The effect of precipitation and dissolution of Si on the thermal diffusivity in the Al-Si alloy system is reported in this study and solution heat treatment followed by aging treatment is carried out to determine the effects of heat treatment on the thermal characteristics. The solution treatment is performed at 535 °C for 4 and 10 h and then the specimens are cooled by rapid quenching. The samples are aged at 300 °C for 4 h to precipitate Si solute. The addition of 9 wt% silicon contents makes the thermal diffusivity decrease from 78 to 74 mm/s2 in the cases of solid solution treated and quenched samples. After quenching and aging, the Si solute precipitates on the Al matrix and increases the thermal diffusivity compared with that after the quenched state. In particular, the increase of the thermal diffusivity is equal to 10 mm/s2 without relation to the Si contents in the Al-Si alloy, which seems to corresponded to solute amount of Si 1 wt% in the Al matrix.