To better understand the material properties of cast aluminum alloys, the mechanical properties of microscopic structures (the eutectic structure and the alpha-Al phase) have been systematically examined using a special nano-indentation hardness test machine. In this approach, a triangular indentation is applied directly to the eutectic structure or alpha-Al phase, and the mechanical properties evaluated through hardness and load-strain relationships. The hardness of the eutectic phase varied, depending on the structural characteristics of the different intermetallic compounds. High values of hardness were obtained structures of the DO(3) type, e.g., Al(5)FeSi, but low hardness for CuAl(2) and Mg(2)Si. The hardness of CuAl(2) and Mg(2)Si had almost the same values as that of the alpha-Al matrix. In addition, from the nano-indentation hardness test, the effects of alpha-Al grain characteristics on the mechanical properties were clarified. The hardness of the alpha-Al grain was linearly related to the grain size and the distance from the grain boundary, where the higher the hardness, the smaller the grain size and the closer to the grain boundary. The hardness level was attributed to the different severity of slip resistance of the atoms during the indentation loading.