A method of fabricating a semiconductor device is disclosed. The method includes forming mandrels over a material layer and forming spacers along sidewalls of mandrels, forming a patterned hard mask to cover a first region, depositing a filling layer in a second region while the patterned hard mask covers the first region. A space between two adjacent spacers in the second region is filled in by the filling layer. The method also includes recessing the filling layer to form a filling block in the space between two adjacent spacers in the second region, removing the patterned hard mask, removing mandrels and etching the material layer by using spacers and the filling block as an etch mask to form material features in the first region and the second region, respectively.