A numerical study was carried out to determine the effects of CO addition on the laminar burning velocity, NOx emission, and extinction strain rate in a premixed CH4/CO/air flame under the lean condition (equivalence ratio of fuel to air φ=0.60-0.80). When more CO was added to the fuel, the laminar burning velocity decreased, which is different from that observed for H2 addition. To explain this, we studied the strong correlation between laminar burning velocity and H+OHpeak concentrations. Results showed that the H+OHpeak concentrations decreased linearly with an increase in CO content. This tendency is in od agreement with that of the laminar burning velocity. For NOx, we observed that increments in CO addition led to a remarkable reduction in the NOx emission. In addition, we investigated the NO formation mechanism and determined the relevant reactions for NO production using a sensitive analysis. The NO concentrations decreased significantly with enrichment by CO and the NO production rate also clearly decreased. We calculated the radial strain rate Srad and discussed the influence of strain rate on lean flame stability with regards to the addition of different CO mole fractions to the fuel. The extinction strain rates indicated that the lean flammability limits were extended by CO addition in some way.