Embodiments relate to a method for manufacturing a semiconductor device that may be capable of obtaining a stable device characteristic by securing an optimal CD of a gate. In embodiments, a method may include forming a gate oxide layer on a semiconductor substrate, forming a photoresist pattern at a first region of an upper portion of the gate oxide layer, forming an insulating layer on the substrate of a second region except for the photoresist pattern, removing the photoresist pattern after a formation of the insulating layer, forming a polysilicon on the substrate from which the photoresist pattern is removed, planarizing the polysilicon to expose the insulating layer in order to form a gate, forming sidewalls at both sides of the gate: and implanting ions in a resulting object using the sidewalls as a mask to form source/drain.