Selective oxidation of methane in CH4-O-2-NO2 was examined using MoO3 catalyst at atmospheric pressure. The selective oxidation was enhanced with addition of NO2 to the reaction system of CH4 and O-2, in analogy with the gas-phase reaction. The initiation reaction was assured to take place between CH4 and NO2. The presence of MoO3 catalyst increased CH2O Selectivity and decreased CH3OH selectivity, compared with those for the gas-phase reaction in CH4-O-2-NO2. Based on variations in reaction of selectivities at several reaction conditions, it could be explained/concluded that formaldehyde was a main product on the MoO3 catalyst and methanol was formed through gas-phase reactions.