Thermal oxidation behaviors of aluminum ion implanted titanium nitride films have been studied in dry oxygen atmosphere. TiN films of approximately 2 μm in thickness were prepared on 18-8 stainless steel (corresponding to AISI 304) substrates by a hollow cathode discharge ion plating. Aluminum ion implantation was performed at energies of 50 and 100 keV with the doses up to 4.5×1017 ions/cm2. Continuous oxidation tests were carried out of TiN films implanted with Al, and oxidation inhibition was evaluated from their mass gain. The structure of the surface layers was characterized by X-ray diffractometer (XRD). X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical bonding states of elements in surface layers of films. The oxidized surfaces of as-deposited TiN films have rutile TiO2 above the temperature of 873 K. However, Al implantation caused the oxidation rate of TiN films to slow down at the initial stage of oxidation. In the case of TiN films implanted with 3×1017 Al/cm2 and oxidized at 1073 K for 2 hours, the Al2p XPS spectrum reveals oxide states as Al2O3, although no oxides were found on XRD patterns. The Al oxides formed on the Al implanted TiN films are considered to improve the oxidation of these films. The initial oxidation behaviors of the Al implanted TiN films are similar to that of TiAlN films deposited by a cathodic arc ion plating.