The present study aims at investigating the effects of microstructure on fracture toughness of two A356 Al alloys. These A356 alloys were fabricated by casting processes such as rheo-casting and casting-forging, and their mechanical properties and fracture toughness were analyzed in relation with microfracture mechanisms. All the cast A356 alloys contained eutectic Si particles mainly segregated along solidification cells, and the distribution of Si particles was modified by the casting-forging process. Microfracture observation results revealed that eutectic Si particles segregated along cells were cracked first, but that Al matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the cast-forged alloys having homogeneous distribution of eutectic Si particles were superior to those of the rheo-cast alloy.