This letter reports the results of thermal fixing of holographic grating in BaTiO 3 crystal, predicating the critical temperature of the formation of complementary ionic grating, the relation between the speed of ionic grating formation and temperature, the relation between the ionic grating formation and the diffractive light oscillation and the effect of different cooling ways on the efficiency of thermal fixing. Thermal fixing of holographic grating with high efficiency is also achieved. We have also observed some new interesting feature of erasing in high temperture and present explanations of the experimental results.