In this study, to investigate the effect of combustion enhancement by irradiating microwave power, NO concentrations have been measured in the combustion experiments using premixed CH4/air flame with and without microwave irradiation. Analyses of NO formation using numerical simulation with a detailed kinetic model have been performed to investigate thermal effect and non-thermal effect of microwave for the combustion enhancement by irradiating microwave power. The experimental result shows that NO concentration in case of combustion with microwave superposition is higher than that in case of normal combustion. In particular, when equivalence ratio is less than 1.0 and Zeldovich NO is a dominant formation mechanism, there has a high increasing rate of NO concentration. The simulation result indicates that NO concentration tends to decrease, when the reaction rate is increased as non-thermal effect of microwave. On the other hand, NO concentration tends to increase, when the combustion temperature is increased as thermal effect of microwave. As a consequence, it is suggested that non-thermal effect of microwave is a small and thermal effect of microwave becomes a dominant factor for the combustion enhancement on premixed CH4/air flame with irradiating microwave power.