A method for forming a hard mask in semiconductor device fabrication comprises: forming first and second patterned material layers on a third material layer, the second patterned material layer only covering the top of predetermined regions of the first patterned material layer; changing a property of exposed top and side portions of the first patterned material layer using the second patterned material layer as a mask, forming property-changed roofs at the exposed top portions of the first patterned material layer and forming property-changed sidewalls with a predetermined width at the exposed side portions of the first patterned material layer; removing the second patterned material layer and portions of the first patterned material layer with exposed tops and an unchanged property located between the property-changed sidewalls, to form the hard mask.