The corrosion behaviors of thixoformed AZ91D alloy immersed in 3.5% NaCl aqueous were studied by using the weight lose method and observed by scanning electron microscopy and analyzed by electron probe micro-analysis.The results showed that corrosion rate of thixoformed AZ91D alloy was higher in initial stages and decreased as the immersion time increased. Compared with the thixoformed alloy, the change of corrosion rate of permanent mould cast (PMC) alloy was smoothly. The observation on the corrosion surfaces showed that the primary αphase spherical particles were first suffered from corrosion during corrosion testing, and then the eutecticαgrains. the composition of primaryαphase particles in the thixoformed alloy was quite uniform, and thus the protective film of the corrosion products on the alloy surface was more intact. The amount of the eutectic β phase in the inter-particle eutectics was relatively higher and the distribution was more uniform, so the protective film consisted by eutectic β phase was also more intact. That is to say the protective role from the surface film on the thixoformed alloy was better than that on the PMC alloy which was the cause of lower corrosion rate of the thixoformed AZ91D alloy.