In this study, A356 Al–Si–Mg cast alloy was aged to various conditions to explore the crack propagation behavior under resonant vibration. The results of all aging conditions indicate that the cracks in the eutectic regions advance by linking with broken silicon particles or decohered particle/matrix interfaces. Crack branching is severe for the peak-aged and overaged specimens in which the cracks tend to propagate in the eutectic regions. These specimens exhibit increasing crack propagation resistance with increasing yield strength and microhardness. For the naturally aged and underaged specimens, the crack propagation resistance also increases with increasing strength and hardness. Crack tortuosity is large for these specimens in which the cracks in α-matrix proceed by meandering along slip bands. Deviation of the test frequency from the resonant frequency occurs when the crack length becomes critical. A change in the aging condition has no apparent effect on this critical crack length.