Methanol is an important industrial material for producing medicine, fuel and resin. It is generally synthesized through the reaction of CO and H-2 in steam reforming of methane. This process is, however, energy consuming; development of a direct method of methanol synthesis that is less energy consuming is required. This paper introduces an experimental study on flow-type hydrothermal oxidation of methane by H2O2. Experiments were conducted under different H2O2 preheating conditions (no-preheating, 120 degrees C, over 270 degrees C) at reaction temperatures from 100 to 350 degrees C. Methane conversion was significantly higher without preheating than when H2O2 was preheated to over 270 degrees C. As a result, methanol yield also increased and showed higher values than previously reported. The increased methane conversion in the absence of preheating was considered to result from elimination of the thermal decomposition of H2O2 that would occur during preheating and the formation of strongly oxidizing OH radicals at the reaction site due to the rapid rise in temperature. This experimental results also suggested that all H2O2 was decomposed to O-2 before it reached the reaction tube when it was preheated at over 270 degrees C.