An effective and selective method for the oxidation of 5-hydroxymethylfurfural (HMF) by using hydrogen peroxide (H2O2) as the liquid oxygen source and activated-carbon-supported ruthenium (Ru/AC) as the catalyst was developed. This reaction system allowed HMF to be oxidized in water under mild reaction conditions efficiently, as it was shown to have lower mass-transfer resistance than gaseous oxygen-assisted oxidation systems. In addition, we could selectively control the components of the oxidation products by adjusting the reaction conditions. We optimized several reaction parameters such as the reaction temperature (75 ��C), time (t=1 or 6 h), base additive (sodium carbonate), and the HMF/catalyst ratio (50 or 10) to attain the desired products in maximum yields. Thus, 5-formyl-2-furoic acid was obtained in a high yield up to 92 % with a HMF/catalyst ratio of 50 and a reaction time of 1 h, whereas 2,5-furandicarboxylic acid was obtained in a maximum yield of 91.3 % with a HMF/catalyst ratio of 10 and a reaction time of 6 h. A possible mechanism for the selective oxidation of HMF was also discussed. We envision that the H2O2-mediated oxidation systems proposed in this study would be of great benefit to other organic oxidation systems.