In an embodiment, a method of forming a gate structure for a semiconductor device includes forming a preliminary gate structure on a semiconductor substrate. The preliminary gate structure includes a gate oxide pattern and a conductive pattern sequentially stacked on the substrate. Then, a re-oxidation process is performed to the substrate having the preliminary gate structure using an oxygen radical including at least one oxygen atom, so that an oxide layer is formed on a surface of the substrate and sidewalls of the preliminary gate structure to form the gate structure for a semiconductor device. The thickness of the gate oxide pattern is prevented from increasing, and the quality of the oxide layer is improved.