Flow distribution of liquid aluminum alloy and its solidification behavior in ADC12 aluminum alloy cam cover for automobile engine were simulated by MAFMA soft at different gate thickness to predict potential defect positions during filling process. The results reveal that gate thickness greatly affect the filling process and distribution of pressure. There exists large temperature-difference and centralized high pressure as well as high velocity impaction around thin-walled gate,resulting in the formation of hot crack. Pressure distribution,entrapped-gas and gas hole defects in the parts are acceptable in condition of gate with thickness of 2.0mm and filling at 50 m/s.