Anti–reducing effect of CaO in barium titanate based ceramic materials sintered at reducing atmosphere and the structure development of the materials has been studied in this paper.The results show: Addition of CaO in barium titanate based ceramic will keep resistivity of the sample sintered at reducing atmosphere at a high level. The resistivity of the materials increase with the increasing of CaO, and the resistivity keep in constant after the amount of CaO excess 3mol%. In the sample with sintering aid of SiO2 and BaO-excess, introducing of CaO may improve the resistivity of the materials obviously. A slight new phase formation was found in the BaO-excess sample, but no new phase formation was found in the non BaO-excess sample. If there is sintering aid of SiO2 in the sample, CaO may accelerate liquid phase film formation at the surface of grains, which will suppress the diffusion of oxygen in the grains to the environment.