A two step deposition process had been used to deposit c axis oriented AlN film on TC4 titanium alloy substrates by mid frequency magnetron sputtering. The surface and cross section morphologies, crystal structure and surface roughness of the AlN films were characterized by scanning electron microscopy (SEM), X ray diffractometry (XRD) and atomic force microscope (AFM). The results show that the full width at half maximum of the X ray diffraction rocking curves around c axis and surface roughness of the AlN films are 4.1° and 2.3 nm respectively，which are prepared on the TC4 titanium alloy substrates with the proposed two step deposition process.The prepared AlN films can be used to fabricate piezoelectric MEMS devices, film surface acoustic wave devices and bulk acoustic wave devices.