In this paper, composite thermal barrier coatings (TBCs) of Al2O3+ZrO2 are deposited on SS304 stainless steel substrates by gas tunnel type plasma spraying. The resultant coating samples prepared with different Al2O3+ZrO2 mixing ratio and thickness are compared in corrosion resistance. A potentiostatic anodic polarization corrosion tester is used to get the corrosion characteristics of coated samples to measure their corrosion resistance. From the polarization curves, corrosion potential and deactivated corrosion current density are obtained and analyzed corresponding to the microstructure of the coatings. The results show that the higher alumina content and thicker coatings, the better the corrosion resistance, which is attributed to the diffusion resistance of coating layers to corrosion reaction. The coating microstructure is characterized using an optical microscope imaging system and measured the coating thickness and porosity. The coating porosity and thickness are believed to be the important factors for the variation on the corrosion behavior of Al2O3+ZrO2 coated SS304 stainless steel substrates.