Coal combustion with O-2/CO2 is promising because of its easy CO2 recovery, extremely low NOx emission, and high desulfurization efficiency. However, there exist many unknowns concerning in-furnace desulfurization in O-2/CO2 pulverized coal combustion, which is much different from that in conventional coal combustion. Research on CaSO4 decomposition aimed at desulfurization in O-2/CO2 pulverized coal combustion is also necessary to investigate the desulfurization behaviors. The kinetics of desulfurization and CaSO4 decomposition were examined in atmospheres of high CO2 and SO2 concentrations. The characteristics and mechanisms of desulfurization in an O-2/CO2 pulverized coal combustion System were clarified by combining experimental results and modeling with the kinetics obtained by us. The effects of sorbent type, residence time. and so on were investigated. It has been identified that the system desulfurization efficiency in O-2/CO2 pulverized coal combustion is increased to about four to six times as high as that of conventional pulverized coal combustion mainly attributed to the following: (1) practical residence time of SO2 is extended and SO2 is enriched inside furnace owing to flue gas recirculation; (2) CaSO4 decomposition is inhibited because of a high SO2 concentration. Their contributions were quantitatively clarified as follows: below 1450 K, the former contributes above two-thirds, whereas above 1500 K, the latter contributes above two-thirds to the overall increase in desulfurization efficiency. The system desulfurization efficiency in O-2/CO2 pulverized coal combustion maintains a high value in a wide range of temperature and particle residence time. These results of a high system desulfurization efficiency, in addition to an extremely low conversion ratio from fuel-N to exhausted NO and an easy CO2 recovery without CO2 separation process, make simultaneous control of CO2/NOx/SOx possible in O-2/CO2 pulverized coal combustion, It can be concluded;that O-2/CO2 pulverized coal combustion is a promising advanced coal combustion technology for the protection of global environment.