The present invention is to provide a method for fabricating a semiconductor device which can minimize the defect density of the substrate, reduce the junction depth of the source/drain, and minimize the leakage current in the source/drain regions by implanting boron ions into the substrate in two steps which are different from each other by implant energy and implant dose.According to the invention, this method of fabricating semiconductor device comprises the steps of forming a gate oxide layer and a gate electrode on a semiconductor substrate or on a semiconductor substrate having N-well; implanting boron ions into the substrate at first and second ion implantation steps, the interstitial point defect region caused by the first ion implantation step overlapping with the vacancy point defect region caused by the second ion implantation step; and activating the boron implanted into the substrate by means of a subsequent thermal process to form source/drain regions.