To study the anti-oxidation mechanism of WS2 films, the pure WS2, and Al doped WS2 composite films were prepared via radio frequency sputtering and the atomic oxygen (AO) irradiation tests were conducted using a ground AO simulation facility. The tribological properties of both films before and after AO irradiation were evaluated using vacuum ball-on-disk tribo-tester. The incorporation of a small fraction of Al dopant resulted in microstructure change from loose columnar platelet with significant porosity for pure WS2 film to very dense structure. In pure WS2 film, WS2 exists as crystalline phase with edge-plane preferential orientation, but nanocrystalline and amorphous phase coexists for the WS2-Al composite film. Even if large amount of AO transported into the interior through the longitudinal pores, the pure film showed good AO irradiation resistance owing to the basal plane of WS2 crystal exhibiting much higher anti-oxidation capacity than the edge-plane. The composite film also had excellent AO irradiation resistance due to the formation of effective thinner WO3 cladding layer in the sub-surface layer. Tribological results revealed that the composite films showed a significantly improved wear resistance, in comparison to the pure WS2 film. Besides, due to the effective AO resistance, the tribological properties of WS2 films remained almost unchanged before and after AO irradiation. (C) 2014 Elsevier B.V. All rights reserved.