Using in situ high temperature X-ray diffraction, the authors investigated the transformations from anatase to rutile at different temperatures (850, 863, 875, 888, 900℃). The experiment results of transition percentage versus time show that the conversion is first-order, and is characterized by an activation energy of 432.788±25.657 kJ/mol. The rate constants based on Arrhenius equation A is 4.847 454×10¹⁵ s^-1. ESEM was used to find out the influence of temperature and time on grain size growth. Two experiment groups, transformation completed samples sintering at different temperatures (888, 900, 930, 960, 1 000℃) for 10 hours and transformation completed samples sintering at 1 000℃ for different periods of time (0, 5, 10, 15, 20 h), were studied. ESEM images show that at 1 000℃, the influence of temperature on grain size growth is not observable and fine crystal form of rutile does not exist. At 1 000℃, grain size grows with increasing time. Fine crystal form of rutile exists after sintering for 10 hours. The relationship between ln D_t (D_t is the average grain size in time t) and time shows that grain size growth has experienced two different periods.