A method for fabricating a semiconductor device including forming a gate insulation layer, a conductive layer for a gate electrode, and an insulation layer for a gate hard mask over a substrate, selectively etching the insulation layer for a gate hard mask and the conductive layer for a gate electrode to expose a first region of the substrate, thereby forming an initial gate line, forming a first insulation layer for an insulation over a resultant structure where the initial gate line is formed, performing a planarization process until the insulation layer for a gate hard mask is exposed, and selectively etching the insulation layer for a gate hard mask and the conductive layer for a gate electrode to expose a second region of the substrate, the second region being not overlapped with the first region, thereby forming a final gate line having a line width smaller than the initial gate line.