The microstructure of the aluminum-silicon (Al-Si) alloy solidified under high pressure was compared with that solidified under normal pressure. High-pressure solidification was performed at 4.0 and 5.0 GPa. The microstructures of the samples were observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The effects of pressure were analyzed by examining the microstructure of the alloy. The observed microstructures show an additional amount of primary a(Al) phase in the Al-13at%Si alloy solidified under high pressure; this indicates the existence of a eutectic point moving toward the silicon direction with increasing pressure. The eutectic cells of the Al-Si alloy solidified under high pressure are evident, and the eutectic silicon phases are presented as curved and near-parallel lamellar crystals, signifying that the growth mechanism of the eutectic silicon phase changed from faceted to non-faceted. The eutectic silicon lamellar phase and the primary b(Si) phase were refined by the applied pressure.