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An effective and selective oxidation of 5-hydroxymethylfurfural (HMF) using hydrogen peroxide (H2O2) as the liquid oxygen source and activated carbon supported ruthenium (Ru/AC) as the catalyst was demonstrated in this work. This reaction system allows HMF to be oxidized in water and mild reaction conditions efficiently since it has relatively less mass-transfer resistance compared to gaseous oxygen-assisted oxidation systems. In addition, we could selectively control the components of oxidation products by adjusting the reaction conditions. We optimize several reaction parameters such as the reaction temperature (75 °C), times (t = 1 or 6 hr), additive base (sodium carbonate), and the ratios of HMF/catalyst (50 or 10) in order to attain the desired products with the maximum yield. The results showed that 5-formyl-2-furoic acid (FFCA) with a high yield up to 92% could be obtained at the conditions of HMF/catalyst = 50 and t = 1 hr while 2,5-furandicarboxylic acid (FDCA) with the maximum 91.3% yield could be obtained at the conditions of HMF/catalyst = 10 and t = 6 hr. A possible mechanism of such selective oxidation of HMF is also discussed. We envision that the H2O2-mediated oxidation systems proposed in this study would be a great benefit to other organic oxidation systems.