The role of oxygen in the heat treatment process of SrTiO3 varistor ceramics has been investigated. It is found that the varistor voltage of SrTiO3 ceramics independent of the sample thickness, and it increases with the heat treatment temperatures increase, that the dielectric property is mainly governed by a highly resistive surface layer. The XPS results of Mn 2p and O 1s suggest that the surface layer was formed by oxygen diffusion and surface chemisorption at grain boundaries during the heat treatment in air. The chemisorption of oxygen in grain boundaries, which leads to the highly resistive surface layer, is the origin of the varistor behavior of SrTiO3 ceramics.