A method of forming metal lines of a semiconductor device includes forming an etch stop layer over a semiconductor substrate over which underlying structures are formed, forming an insulating layer over the etch stop layer, etching the etch stop layer and the insulating layer to form trenches through which the underlying structures are exposed, shrinking the insulating layer by using a thermal treatment process in order to widen openings of the trenches, and filling the trenches with a conductive material.