Abstract: Fracture mechanism of two-phase alloy is discussed based on the observation of the fracture process of Al-Si hypereutectic alloy by using optical microscopy.It is found that the fracture of Al-Si hypereutectic alloy occurs firstl on the nook of primary silicon and with the increase of outer forces,the crack expands to the internal of primary silicon and the direction changes at the phase boundary and expands along the phase boundary.Analysis suggests that the bonding strength of the phase boundary is the weakest and crack propagation occurs preferably in the weak for Al-Si two-phase alloy.