The electrical property and microstructure of SrTiO, capacitor-varistor ceramics sintered in different atmospheres were investigated in the present study. The electrical experimental results show that samples sintered in strong reducing atmosphere of 25%N2+75%H2 and heat-treated in air have a low-resistive core with a high-resistive surface layer around it which is named surface effect. The XPS results reveal that surface effect results from the diffusion process of oxygen during heat-treatment.