Permanent mold and semi-liquid die A356 cast alloys were used to examine the roles of microstructures and aging conditions on fatigue crack growth. HIP treatment to the A356 alloy generates substructure like dendritic arm boundaries as well as reduction of pores, which improves fracture elongation and fatigue fracture toughness. The similar substructure occur at primary α-Al and inter Si particles of semi-liquid die cast, too. Fracture elongation of HIPed permanent mold cast is comparable to that semi-liquid die cast, the fatigue crack growth is faster than in semi-liquid die cast. Plastic hardening occurs around fatigue crack flank, which decreases fatigue crack growth rate, and such effect appears highly in under aged alloy.