The laminar flame speed of natural gas/carbon monoxide fuel mixtures is studied by experimental and numerical methods. Experimental measurement shows that with the increase of the volume fraction of natural gas, the maximum laminar flame speed of natural gas/carbon monoxide fuel blend first increases and then decreases, and the peak value is obtained at 0.748 m/s for 8% natural gas in fuel blend. Numerical simulation reveals that the nonmonotonic variation trend of the laminar flame speed with natural gas content is mainly attributed to the transition of the dominant chemical kinetic steps.