Positron annihilation spectroscopy is a nondestructive evaluation method which is sensitive to study the microdefects of materials. In this article, hypereutectic Al-20Si-0.35RE (wt.%) pre-alloyed powders was synthesized by using the water atomization method. The pre-alloyed powders were subsequently hot extruded with pressure of 250~500 MPa at 300°C. The effect of pressure of hot-extrusion on the evolution of microdefects in Al-20Si-0.35RE alloy was investigated by positron annihilation spectroscopy, electrical conductivity tests and hardness tests. It was shown that the accretion of pressure led to an apparent increase in conductivity and microhardness. For all samples, the hot-extrusion causes a pronounced change in positron annihilation related parameters signaling the variation of microdefects. The results of hardness tests and the electrical conductivity tests are varied following with the positron annihilation spectroscopy. Possible mechanisms of the influence of hot-extrusion on the behavior of microdefects are discussed.