The microstructural evolution of nanocrystalline anatase TiO2 during high-energy planetary ball milling was studied. The results show that mechanical activation induces the transformations of nanocrystalline TiO2 from anatase to srilankite and rutile at room temperature and under ambient pressure. As the milling time increases, more anatase powder transforms to the srilankite and rutile phase, and the particle size of the powder decreases. There is no indication of the formation of amorphous phase during ball milling.