CrAlN thin films were prepared by using the reactive DC unbalanced magnetron sputtering method from the single alloy target on a silicon substrate. The effect of annealing temperature in the air which ranges from 500℃ to 900℃ for 1 h on phase structure, film composition, surface morphology, microstructure, and hardness was investigated by XRD, EDS, FE-SEM, and Nanoindentation techniques, respectively. The high-temperature (up to 900℃) oxidation resistance of the thin film was also evaluated. The result shows that solid solutions of (Cr,Al)N with (111), (200), and (220) planes for the as-deposited film and no oxide phase were found after annealing with different temperatures. The O content slightly increases with an increase in the annealing temperature with various Cr, Al, and N contents found by the EDS. From the FE-SEM result, as increased annealing temperature, the evolution of cross-sectional morphology from dense to compact columnar structure was exhibited but the oxides layer was not detected. These results concluded that the as-deposited thin film showed good oxidation resistance when annealed in air at an elevated temperature reaching 900℃. Moreover, the film’s hardness decreased from 61.19 GPa to 50.11 GPa with increasing the annealing temperature observed by the Nanoindentation technique.