The invention relates to an MOS (Metal-oxide Semiconductor) transistor and a making method thereof. The MOS transistor comprises a silicon substrate, an oxide layer arranged on the silicon substrate, top silicon arranged on the oxide layer, a first gate through hole, a second gate through hole, a gate dielectric, a gate, a source / drain electrode extension region and a source / drain electrode, wherein the top silicon consists of a first p-type single crystal silicon layer, a first n-type single crystal silicon layer, a second p-type single crystal silicon layer and a second n-type single crystal silicon layer, the first gate through hole is arranged in the center of the first p-type single crystal silicon layer, a second gate through hole is arranged in the center of the second p-type single crystal silicon layer, the gate dielectric and gate are arranged on the inner wall of the first gate through hole, the inner wall of the second gate through hole and the second n-type single crystalsilicon layer, the source / drain electrode extension region is arranged in a source / drain electrode region at both sides of the gate, the source / drain electrode is arranged in the source / drain electrode region at the both sides of the gate, and corners of the first n-type single crystal silicon layer and the second n-type single crystal silicon layer are smooth. The invention enhances the utilization ratio of the area of a chip and solves the corner effect caused by strong corner current.