The decomposition behavior and mechanism of calcium sulfate in O-2/CO2 pulverized coal combustion were studied in an entrained flow reactor. A reaction rate expression correlating the influence of various factors was proposed for CaSO4 decomposition and it is able to predict CaSO4 decomposition satisfactorily. Under the conditions investigated, the decomposition of CaSO4 was found to be a regime of chemically controlled shrinking core reaction. A CO2-rich atmosphere enhances CaSO4 decomposition in absence of oxygen. CaSO4 particles have catalytic effect on formation of CO from CO2. A high SO2 concentration inhibits CaSO4 decomposition. The kinetics of CaSO4 decomposition has obvious dependence on experimental facilities and conditions, whereas the activation energy has much lower dependence. The kinetics derived in this work is more appropriate for investigating desulfurization in O-2/CO2 pulverized coal combustion because an entrained flow reactor has a much closer condition to that in O-2/CO2 pulverized coal combustion than a TGA.