To reveal the interfacial phase structure and its effect on mechanical propertie s of Ti_Si_N composite films, x_ray diffractometry, high_resolution transmission electron microscopy, Auger electron spectroscopy, and microhardness tester were employed to investigate theinterfacial phase structure and mechanical pro perties of magnetron sputtered Ti_Si_N composite films. A series of TiN/Si3 N4 multilayered films in nanometer scale were also prepared and characte rized for comparison. The results indicated that Ti_Si_N composite films formed a structure of nanocrystalline TiN surrounded by Si3N4 int erfacial p hase. In the Ti_Si_N composite film with lower silicon content, the Si3 N4 interfacial phase with a thickness of less than 1 nm crystal lized and formed a coherent interface with TiN nanocrystals, leading to an enhancement in the hardness of the film. Whereas, in the composite film of larger silicon cont ent, Si3N4 phase existed as amorphous and resulted in a de crease in hardness. Our research indicated that the crystallization of Si3N4 i nterfacial phase was essential to obtain a high hardness in the Ti_Si_N composit e films, and the strengthening mechanism of the composite films appeared to be the same as TiN/Si3N4 multilayered films.