In a method of forming MOS transistor, a gate structure is formed on a substrate and a first spacer layer is formed on the substrate conformal to the gate structure. A second spacer layer is formed on the first spacer layer. A second spacer is formed on the first spacer layer corresponding to a sidewall of the gate structure by partially removing the second spacer layer from the first spacer layer. Impurities are implanted in the substrate by an ion implantation process using the gate structure including the first spacer layer and the second spacer as an ion implantation mask to form source/drain extension regions at surface portions of the substrate around the gate structure.