Permanent mold cast (PMC) AJ62 magnesium alloy exhibits a fine-grained microstructure in the thin section and a coarse-grained microstructure in the thick section. Microstructure of the PMC AJ 62 alloy was analyzed by using the Scanning Electron Microscopy (SEM). Potentiodynamic polarization experiments were performed to investigate the corrosion resistances of the PMC AJ62 alloys in salt solutions and engine coolant. The corrosion behaviors in the fine- and coarse-grained AJ62 alloys were compared. The results show that the AJ62 alloy with fine microstructure presents enhanced corrosion resistance.