In an exemplary embodiment of the invention a method of forming a gate oxide layer of a semiconductor device uses deuterium gas. The method includes introducing a semiconductor substrate, and depositing an insulating layer on the semiconductor substrate by supplying an oxidation reaction gas and a deuterium gas to the semiconductor substrate. Thus, a high quality gate oxide layer can be formed and resistance to degradation from the hot carrier effect can be improved. Further, when the method is applied to a tunnel oxide layer process of a flash memory, problems such as an increasing dispersion of the threshold voltage can be mitigated.