Thin ZrO2/Al2O3 films that consisted of alternating monoclinic ZrO2 nanolayers and amorphous Al2O3 nanolayers were deposited on a tool steel substrate using Zr and Al cathodes in a cathodic arc plasma deposition system, and then oxidized at 600-900 degrees C in air for up to 50 h. The ZrO2/Al2O3 films effectively suppressed the oxidation of the substrate up to 800 degrees C by acting as a barrier layer against the outward diffusion of the substrate elements and inward diffusion of oxygen. However, rapid oxidation occurred at 900 degrees C due mainly to the increased diffusion and subsequent oxidation of steel as well as the crystallization of amorphous Al2O3.