A356 is the aluminum casting grade which has compositions that combines outstanding casting characteristics with excellent properties after heat treatment. Mechanical properties of A356 can be improved by reducing of secondary dendrite arm spacing (SDAS), precipitation hardening, and the interaction effect of both. It is generally accepted that dendrite arm spacing and fine distribution microstructure are related to each other and they also affect the precipitation hardening in a way that smaller SDAS results in shorter time required to obtain a satisfactory degree of solution of the undissolved or precipitated soluble phase constituents and to achieve good homogeneity. Minor addition of Sb was successfully used in reducing the SDAS in previous work. However, the effect of Sb addition on age hardening has not been investigated, especially in a high cooling rate condition. In this research, effects of minor addition of Sb on SDAS, age hardening and mechanical properties; i.e. hardness and tensile properties, are reported. It was found that Sb addition did not clearly affect SDAS at the high cooling rate, i.e. as in permanent mold casting process. Moreover, we found that the addition of Sb into A356 also lowered mechanical properties.