CrN films were deposited onto (100) Si wafers by cathodic arc plasma deposition. After that, the films were annealed between 400 and 1200 degreesC for 2 h in air, N-2, and N-2/H-2 = 9, which possess dramatically different PN2 and Po,. XRD results showed that oxidation of CrN films occurred above 700 degreesC in all gases but the relative amount of the resultant oxide Cr2O3 decreased with rising (PN2/PO2) ratio in the gases for a given temperature. The driving force for oxidation of the nitride is the Gibbs free energy changes in the oxidation reaction. Meanwhile, the beta -Cr2N phase appeared at 500 degreesC, diminished it 700 degreesC, and showed up again at 1100 degreesC under all atmospheres. The reason for the presence of beta -Cr2N at temperatures above 1100 degreesC is that Cr2N is thermodynamically more stable than CrN in the high temperature range, as analyzed front thermodynamics. The phase transforming from CrN to Cr2N in the low temperature range is possibly due to the large stress relaxation occurring in the film during annealing, as observed in the in situ stress measurements.