In this study, the combustion experiments using premixed CH4/air flame with and without the irradiation of a microwave have been carried out to investigate the effect of combustion enhancement by irradiating microwave power under equivalence ratios between 0.7 and 1.1. When the flame is radiated by a continuous wave microwave field of approximately 100 W, the consumption rates of CH4 and O-2 increase and the generation rate of CO2 increases at all equivalence ratio conditions. The generation rates of H-2 and CO increase at equivalence ratio 1.1. As a result, it is shown that the combustion flame is enhanced by irradiating microwave power. The optical emission intensities of OH radical and CH radical, which play an important role on combustion process, increase at 10 mm from the nozzle and they decrease at 25 mm from the nozzle, when the flame is radiated by a microwave. As a consequence, it is indicated that the combustion reaction is enhanced and the flame length is shortened by irradiating microwave power.