A method of fabricating a semiconductor device having a gate spacer layer with a uniform thickness wherein a gate electrode layer pattern is formed on a substrate and ion implantation processes of respectively different doses are formed on side walls of the gate electrode layer patterns in respective first and second regions of the substrate. A first gate spacer layer is formed on the gate electrode layer pattern where the ion implantation process is performed. A second gate spacer layer is formed on the first gate spacer layer.