Among the several methods to prepare graphene oxide (GO) from graphite by oxidation, Hummers method has been the most widely used. Since Hummers method was first introduced about 60 years ago, many researches have tried to reveal the effect of each reagent on the final properties of GO. In all Hummers methods including the modified ones, sulfuric acid and potassium permanganate are used for oxidizing graphite and exfoliating oxidized graphite, and water and hydrogen peroxide are used for removing the residual manganese ions. However, the effects of hydrogen peroxide have been sometimes ignored during synthesis since hydrogen peroxide has been understood as simply reducing the residual manganese ions such as permanganate and manganese dioxide. For this reason, the quantity of hydrogen peroxide used for Hummers method varies. Here we demonstrate that the quantity of hydrogen peroxide added in the final stage strongly affects the final properties of GO. Experimental evidence is introduced revealing that chemical decomposition of GO with slight reduction accompanies the addition of hydrogen peroxide. The possible degradation mechanism by hydrogen peroxide in Hummers method is proposed in the last part of this study.