The influence of T6 heat treatment on the microstructures and aging precipitation in the grain-refined A356 alloys were investigated. Experimental results showed that, when T6 heat treatment was performed, the average values of mean diameter, aspect ratio, and roundness of silicon particles in the grain-refined specimens were decreased to 2.26~3.14 μm, 2.06~2.2, and 3.1~4.2, respectively. DSC curves revealed that three kinds of precipitates, i.e., β″, β′, and β, were observed in both unrefined and grain-refined A356 alloys. The exothermic peaks in the grain-refined A356 alloy shifted to high temperatures, suggesting the nucleation of β″ precipitates was decelerated. This could be ascribed to the minor solution of Ti or B elements during solid solution treatment.