Based on the strong inhibition of SiC particles on the grain growth of the Al2O3 matrix, a mixture powder of alpha-Al2O3 and SiC was used as feedstock to deposit the nanocomposite coatings by plasma spraying. To determine the effect of plasma spraying gas on microstructure and mechanical properties of Al2O3-SiC nanocomposite coatings, primary Ar gas and secondary H-2 gas were chosen, which have an effect on temperature and velocity of the flying powders at the time of impacting on substrate. The X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were used to characterize the microstructures of these coatings. Surface roughness was measured also.