The additinal stabilizing heat treatment was employed in order to remove the interference which there still exist precipitation of secondary phase that cause volume of the piston unstable during the piston working. The thermal expansion coefficient of eutectic Al-Si alloy piston was measured by laser-interfereatial expander from room temperature to 400C Investigation of the influence of rare earth metals on the thermal exponsion coefficient was also carried. The results demonstrate that the accuracy and reliabity are greatly improved.