The oxidation of coal char/CaS mixtures was studied with a fixed bed reactor in the temperature range 700 similar to 900 degrees C. It was found that oxidation of coal char/CaS experienced a process from reducing atmosphere to oxidizing atmosphere according to the change of CO concentration which was caused by partial combustion of coal char. The amount of SO2 released in a reducing atmosphere was more than double that released in an oxidizing atmosphere. It was regarded that the whole process of oxidation of coal char/CaS mixtures was dominated by different reactions, The amount of SO2 released at an oxidation temperature of 900 degrees C was largely increased compared with that at 800 degrees C, because the oxidation of CaS was more enhanced at 900 degrees C, XRD analysis also verified that the amount of CaSO4 produced at 900 degrees C was more than that at 800 degrees C, It was revealed that the further oxidation of CaS, especially at low temperature, might be blocked by the reaction product of CaSO4 which prevented the diffusion of O-2 into the inside of the unreacted core of CaS particle due to a larger molar volume of CaSO4.