Continuous experiments on sulfurization and oxidation of CaO particles were carried out in a thermobalance with a quartz spring. The effects of temperature, O-2 partial pressure, initial CaS content and particle diameter on oxidation reaction of CaS particles are investigated under advanced pressurized fluidized bed combustion conditions. Our experimental results show that the CaS particles with initial CaS content of 30 mol% and diameter below 0.9 mm are completely oxidized in atmospheres with Oz partial pressures of 1.01 kPa and 5.07 kPa at temperatures of 1,203 and 1,253 K. For 0.2 and 0.5 mm particles with initial CaS content of 80 mol%, the complete conversion is achieved at 1,253 K in atmospheres with O-2 partial pressures of 1.01 to 21.3 kPa. It is found that CaS particles are first oxidized to form CaO through the reaction (1): 2 CaS + 3 O-2 --> 2 CaO + 2 SO2; and then the releasing SO2 gas through CaO layer reacts with the diffusing O-2 to form CaSO4 product layer through the reaction (2):2 CaO + 2 SO2 + O-2 --> 2 CaSO4. Due to the higher molar volume of CaSO4 than those of CaS and CaO, the diffusing rates of SO2 and O-2 gases passing through the CaSO4 layer become slower, consequently the oxidation reaction declines and finally ceases. A kinetic study on the initial reaction rate shows that reactions (1) and (2) are both the first order with respect to oxygen and have activation energies of 426 kJ/mol and 52 kJ/mol, respectively.