BaTiO3 films on Ti substrate were fabricated by alternative current(AC) and direct current(DC) micro arc oxidation (MAO). Microstructures of films were investigated by means of SEM, XRD and TEM. The results show that the amorphous phase and primitive cubic phase are the main phases in the films prepared by AC MAO. Even after being annealed at 1 200 ℃ for 8 h,only a few tetragonal phases can be observed in films prepared by AC MAO. However, tetragonal BaTiO3 phase can be produced by DC MAO directly. In the films prepared by DC MAO, a mixture of cubic phase and tetragonal phase is formed. After sparking spacious distribution, sparking duration and temperature gradient near sparking sites were taken into account, and a mechanism of synthesis of tetragonal BaTiO3 phase by DC MAO was proposed.