The mechanism of damage evolution and fracture in A357 casting alloys was investigated by in-situ scanning electron microscopy(SEM) tensile testing.Different microstructures of A357 casting alloys were produced by eutectic Si modification and T6 heat treatment.It is shown that microcracks in these alloys are predominantly formed in eutectic Si particles.Large and elongated eutectic Si particles in unmodified alloy show the greater tendency to cracking,whereas cracking of small and round eutectic Si particles in Sr modified and T6 heat treated alloys is relatively lag.The crack mainly propagates along the broken eutectic Si particles in unmodified and Sr modified alloys or along the deepened shear bands in T6 heat treated alloy with accumulating the applied strain.The results were discussed in terms of Weibull statistics and the fracture models were established. The mechanism of damage evolution and fracture in A357 casting alloys was investigated by in-situ scanning electron microscopy (SEM) tensile testing. Different microstructures of A357 casting alloys were produced by eutectic Si modification and T6 heat treatment. It is shown that microcracks in these alloys are predominantly formed in eutectic Si particles. Large and elongated eutectic Si particles in unmodified alloy show the greater tendency to cracking, while cracking of small and round eutectic Si particles in Sr modified and T6 heat treated alloys is more lag. along the broken eutectic Si particles in unmodified and Sr modified alloys or along the deepened shear bands in T6 heat treated alloy with accumulating the applied strain. The results were discussed in terms of Weibull statistics and the fracture models were established.