(Ti,Al)N coatings possess superior hardness, good thermal and chemical stability. To further study the oxidation resistance, the arc-deposited AlTiN bulk material was collected from the substrate shutter of a commercial coating system deposited for over 100 batches. The thermal behaviour of the materials was studied by TGA/DSC with different annealing temperature or different atmospheres. The results show that AlTiN bulk material annealed in nitrogen exhibits the denser structure and thermal stability up to 1000°C. The oxidation resistance of the AlTiN can reach to 800°C. Annealed at 600 ~ 700°C proceed the recrystallization process which denser the structure. New phase of TiO2is formed by incorporate oxygen in exchange of nitrogen. nanoscaled 1D wire formation occurred at the 1000°C for 3 hours in air.