Anatase (TiO2) is transformed into rutile via brookite within 1 h by room temperature grinding using a planetary ball mill. This polymorphic transformation of anatase leads to the mechanochemical synthesis of the crystalline CaTiO3 from the mixture with CaO easier than from the CaO and rutile mixture. The amount of CaTiO3 in the ground product increases with improving its crystallinity as the grinding progresses. They reach a 20-nm size by about 5 h of grinding, and the grain boundary and lattice fringe appear clearly as the grinding progresses.