The fracture mechanisms of E-A356 alloys (T5 treatment) have been investigated by means of in situ SEM under nonaxial tension loading. It is found that the crack initiated at the casting defects, such as at the gas or the shrinkage pores, because of the debonding of the silicon particles from the Al matrix. The crack propagation was mainly in the Al matrix and along the matrix/particle interface secondarily. The crack growth could be influenced by the eutectic silicon particles, when it encountered the Si particles, the growth direction would be changed and turned toward the weaker areas. The fracture resistance or the mechanical properties of the E-A356 alloys will be improved by means of reducing, eliminating gas or shrinkage pores and increasing the interface bound strength of the eutectic silicon particles with the Al matrix.