The micro_structure of natural rutile heated and annealed under argon atmosphere was studied by means of X_ray Diffraction (XRD) and Transmission Electron Microscopy (TEM). The results indicate that the lattice of heated rutile specimens was expanded, and ilmenite appeared when the heating temperature reached 900℃ and 1000℃. Like rutile, ilmenite is a kind of natural semiconductor mineral. It grew in the direction at a 31°angle with rutile's (010) planes and formed dual coupled semiconductor with rutile. The results of the UV_vis test indicate that the appearance of ilmenite in the rutile sample can effectively enhance the photocatalytic activity of the specimens.