The apostle Paul likely died of decapitation due to a beheading by the Romans. While there are no definitive records of Paul's death, decapitation is the commonly accepted reasoning of death.
It is believed that Paul was beheaded in Rome under the rule of the emperor Nero. He was beheaded after he returned from his fifth missionary journey and was likely beheaded due to his involvement with missionary activities. The beheading may have also had to do with his involvement in the burning of Rome. Although Paul did not have major involvement in the burning of Rome, Nero executed all people that had anything to do with the burning of the city.
The time in which Paul's death likely occurred was a tumultuous time for Rome. The emperor committed suicide soon after the beheading of Paul occurred. It is noted in several biblical dictionaries that Paul spent a long amount of time in jail prior to the time that his death is believed to have occurred. The records on file show that Paul was unhappy in jail and was tired of being cold in a cell. This was only a few months prior to May of 68 A.D., around the time that Paul was beheaded.