The relationship between the particle diameter and conversion during combustion of petroleum cokes with different ash contents was examined using a thermogravimetry analyzer and a captive particle imaging apparatus. Cross sections of quenched particles during combustion were analyzed by EPMA. A thin ash layer of high porosity is generated around the unreacted core of the particle, and it was found that the diameter change of the unreacted core can be explained by the Modified Sharp Interface Model. In addition, the kinetics of the reaction order of Petroleum cokes with oxygen was determined on the basis of this model.