Properties of the CaS produced from sulfidation of limestone and its effect on sulfidation rate were investigated by using TG, BET and SEM techniques. CaS produced from limestone had an initial specific surface area of about 6.1x10(3) m(2).kg(-1) with grain size about 0.25 mu m, but fell to 0.6x10(3) m(2).kg(-1) after sulfidation of 40 minutes due to CaS sintering. The CaS sintering led the product layer to shrink and lose porosity, and in turn to affect H2S gas diffusion in to the reaction surface. Conversion of 5 mu m sizes particle only reached about 45% at sulfidation of 25 minutes. An unsteady state product layer diffusion model was developed for describing a gas-solid reaction with a shrinking product layer. In the product layer, the effective diffusion coefficient, D-es(r) was varied with radius r and also with time t. The D-es(r) was determined by porosity changes of the product layer. Good agreement was noticed between calculation and experiment results. According to this model, conversion of 100% is possible obtained by sulfidating limestone particle smaller than 5 mu m.