The effect of thickness on transformation temperature of the NiTi thin films has been studied by X-ray diffraction and X-ray photoelectron spectroscopy. Results show that the crystallization temperature for 3?μm-thick film is higher than that for 18?μm thick film at the same growth temperature and post annealing. With the substrate temperature increasing, the start temperature （As） of austenite phase is lowered after annealing at 763 K for 1?h. There is an oxide layer （TiO2） on the film surface, which prevents the Ni atom from coming onto the surface. There is an oxide layer of a mixture Ti2O3 with NiO on the film /substrate interface. The oxide layers affect the transformation temperature by changing the Ni atomic content in the interior of the film.