Aluminum alloys and castings are provided that have excellent practical fatigue resistances. The alloy includes, based upon 100 mass %, 4-12 mass % of Si, less than 0.2 mass % of Cu, 0.1-0.5 mass % of Mg, 0.2-3.0 mass % of Ni, 0.1-0.7 mass % of Fe, 0.15-0.3 mass % of Ti, and the balance of aluminum (Al) and impurities. The alloy has a metallographic structure, which includes a matrix phase primarily of α-Al and a skeleton phase crystallizing around the matrix phase in a network shape. The matrix phase is strengthened by precipitates containing Mg. Because of the strengthened matrix phase, and the skeleton phase that surrounds it, the castings have high strength, high fatigue strength, and high thermo-mechanical fatigue resistance.