The aging behavior of an A356+1%Cu alloy was examined using hardness, electrical conductivity measurements and transmission electron microscopy (TEM). The thermal fatigue test was also performed for the aged alloy. The hardness increases with aging time at examined temperatures of 160 to 230 C and then decreases to show peak hardness. The corresponding electrical conductivity increases with aging time at all temperatures. The TEM microstructures show that needle-shaped precipitates are formed and grow in their elongated direction with aging time. The increase of electrical conductivity indicates that the volume fraction of precipitates increases with aging time. The thermal fatigue life of the over-aged specimen was two times longer than that at the peak-aged one. The fracture surface of the over-aged specimen was smoother than that at the peak-aged one. The tortuous by propagated path in the peak-aged specimen was originated in the predominant crack propagation through fractured eutectic Si particles. The above difference of crack propagation behavior between peak-aged and over-aged specimens is discussed based on the size difference of precipitates.