CrAlSiN hard coatings were fabricated on the Si substrate from metallurgical Cr0.45Al0.45Si0.10 alloy target by reactive r.f. magnetron sputtering. The oxidation resistance of CrAlSiN coatings was investigated after annealing at temperatures between 900 and 1100°C for 1 hr in air. The phase identification and microstructure of CrAlSiN coatings after heat treatment were analyzed by X-ray diffraction (XRD). The hardness of CrAlSiN coating after heat treatment at 900oC for 1hr in air is slightly decreased from 30.2GPa to 28.3±1.3GPa, which was caused by the thin oxide formation on the surface of the film. The microstructure of CrAlSiN coating after heat treatment at 1000oC from 1 hr analyzed by TEM revealed two types of layer feature, including the nanocrystalline grain embedded in the Al-riched amorphous layer and reaction interface with relative high content of Si.