As commander of the 2nd division of the Whitebeard Pirates, Ace had authority over the lower-ranking subordinates. In addition, as one of the top commanders of the Whitebeard Pirates, he was one of the strongest and most highly-regarded pirates on board under Whitebeard himself. At one point, while being captain of the Spade Pirates, he was even asked to join the Shichibukai but refused. Ace was strong enough to be acknowledged by the Emperors Shanks and Blackbeard, with the former saying he had earned his spot as Whitebeard's second division commander and the latter wishing Ace had joined his crew and emphasizing how his own formidable crew was nowhere near strong enough to combat Ace at the time, speaking volumes of Ace's formidable strength.
Luffy commented on how he never defeated Ace in a single fight, despite Luffy already having the power of the Gomu Gomu no Mi (Ace did not consume the Mera Mera no Mi until after he left: Luffy was surprised by Ace's new power when Ace stopped Smoker from capturing him in Nanohana). Ace was trained by Garp, who had hoped to turn him into a fine Marine, and possibly Dadan, the two of whom were responsible for training Luffy to the level of power he held before becoming a pirate.
Ace also held a fearsome reputation. It was revealed that Ace received an invitation to join the Shichibukai while he was still captain of the Spade Pirates, though he refused the offer. After he joined the Whitebeard Pirates, a civilian was seen commenting on how Ace's strength was supposedly "outta this world". The World Government granted Blackbeard the position of Shichibukai after he successfully defeated and captured Ace. His overall reputation and abilities had earned him a bounty of 550,000,000 prior to his capture and death.
According to Sengoku, Ace had especially immense potential. He described that it was Ace's rapid rise in power and fame that made the World Government realize he was Gol D. Roger's son. He went on to say that in time, Ace could potentially become the next Pirate King if he continued on, which was the most important reason why the World Government was willing to start a war with Whitebeard.
He was also a capable navigator, as he was seen wearing a Log Pose on his left wrist, while travelling island to island in search for Luffy and Blackbeard on a simple raft. This also shows he was a skilled tracker.
Ace's willpower was strong enough to withstand Shanks' outburst of Haoshoku Haki.
Aside from the powers granted by the Devil Fruit he consumed, Ace possessed immense physical strength and endurance which he attained from the extremely rigorous training in the harsh environment of Mt. Colubo mainly from hunting and fighting wild beast since childhood. His incredible physical prowess were supplemented by his extraordinary fighting skills. Even in his early childhood, he was able to beat some local thugs who spoke ill of his father "half to death" and during his first meeting with Luffy, he was also shown sitting on top of a huge bull he killed with his staff. He could also easily break a giant tree with a single kick. In his childhood, before he acquired the Mera Mera no Mi, Ace was able to best Luffy in every single sparring match, despite Luffy's untrained Gomu Gomu no Mi abilities.
Ace demonstrated his strength during his battle with Blackbeard: he was able to take Blackbeard's punches, which were so powerful that the first made Ace cough up blood and the second nearly broke his neck, and retaliate quickly afterwards. Van Augur noted that, as expected of the Whitebeard Pirates 2nd division commander, Ace's basic combat skills were quite formidable even without his Devil Fruit powers.
He fought equally with Jinbe in the past; they nearly killed each other in a fight that lasted for five days straight. Ace then survived a battle with Whitebeard that took place immediately after, showing immense stamina and durability. He was one of the fewpeople to be able to take a "baptizing" in boiling water in Impel Down without flinching, which is considered an impressive feat.
Ace ate the Devil FruitMera Mera no Mi, a Logia-type fruit that allows him to become the element of fire and control the flames from his body, giving him his reputation as "Fire Fist Ace". Ace displayed his immense power by destroying a handful of Baroque Works ships with ease. His main style is to launch columns of fire by punching, with his signature "Fire Fist". He is also immune to normal physical strikes, all of which will simply phase through his fire-composed body.
He traversed the seas in a one-man boat that seems to have been specifically designed for him, propelling it with flames from his feet.
Ace also had practical uses for his Devil Fruit abilities, as he would heat up any fish he would find in the seas for his crew when they had no food to eat.
He skillfully sent his boat underwater as he leaped over the Billions fleet, timing it precisely so that it emerged underneath him just before he hit the water on the other side. Later after his death, it was revealed that Ace's burning fists also had the power to annihilate entire cities.
Ace's Haki had been described by Shanks and Benn Beckman to be fiery in nature, to the point that Ace’s very presence was able to stop blizzards on an island for as long as he was there. As such, when he turned up on Drum Island, no snow fell at all on the island for an entire day.
Ace possessed Haoshoku Haki, which he initially had no conscious control over. He first demonstrated the ability at 10 years of age when he rendered the Bluejam Pirates except for Bluejam unconscious after an outburst of rage. He later gained knowledge of the power, as he recognized it when Luffy used it at the Battle of Marineford.
He was never seen using this Haki apart from during his childhood, save for in the video games (such as during the activation of his Kizuna Rush in Pirate Warriors 3).
It was stated in One Piece novel A that Ace could use Busoshoku Haki. Upon awakening it, he was capable of fighting on equal grounds with Vice Admiral Draw with Busoshoku Haki. However, it is not known how proficient he was in using it.
Ace carries around a knife but is never seen using it. However, in flashbacks of attempts to assassinate Whitebeard, Ace was shown using various weapons, including an axe and a knife. As a child, he used a long bo staff made from pipe as a weapon.
In One Piece: Burning Blood, Ace (Strong World) wields two twin double-barreled firearms (which resemble a cross between a pistol and sawed-off shotgun) which he uses as part of his Twin Gun & Crossfire special attack.