Nano-TiO2 was prepared with butyl titanate as a precursor by sol-gel method. The samples were analysed by TG-DTA, X-ray diffraction, TEM and so on to assess the effects of different temperatures on the crystal structure, grain size, and microstructure of nano-TiO2. Meanwhile, the catalytic effect of heat treatment temperature on the degradation performance of TiO2 to methyl orange was investigated. The dynamic process of grain growth was preliminarily analysed by Eastman’s particle growth theory. The result shows that TiO2 particle size gradually increases with the heat treatment temperature. At 450 to 550°C, the grain is mainly anatase phase, a mixture of anatase and rutile phase was found at 650°C (mass ratio A:R = 9:1), and the degradation rate of nano-TiO2 on methyl orange reaches 97.75%. When the calcination temperature exceeds 850°C, TiO2 particles almost entirely are composed of rutile phase, and the photocatalytic activity decreases significantly. At 730°C, half of the crystalline TiO2 is transformed from anatase to rutile form. The apparent activation energies of the anatase and rutile crystals of nano-TiO2 are 18.15 kJ/mol and 42.56 kJ/mol, and the fastest grain growth occurs at 546°C and 1280°C respectively.