The intemal diffusion effect on selective hydrodesulfurizatlon(HDS)of FCC gasoline was evaluated by using different particle size CoMo/γ-Al2O3 catalysts(the actual-size catalyst and crushed catalyst)in a hlgh-pressure fixed-bed reactor．Evaluation results showed that significant internal diffu sion effect，presenting in the actual-size catalysts，could inhibit the HDS of thiopbenes(Ts)，and tlle Is with little or no stefic hindrance effect suffered a stronger internal diffusion effect，so the inhibidon intensity inereased in the order of T>MTs>DMTs>3 CTs>4CTs：the intemal diffusion effect on hydrogenation(HYD)of olefins might be neglected，however，the crushed catalysts still showed lower HYD activity，which might be due to a relatively shorter diffusion path of olofins in the crushed catalyst compared tO the actual-size catalyst，and tllus tlle olofins had less opportunity to be hydrogenated．More studies are much needed to weaken the internal diffusion effect for much improved efficiency in selective HDS of FCC gasoline．