Combustion of n-heptane usually yields unburned hydrocarbon and carbon monoxide, which are environmentally harmful. As a result, improving the combustion efficiency is much more important. In the work, the effect of methane addition on the premixed n-heptane/air combustion (0.1 MPa and 400 K) was experimentally and numerically investigated. The equivalent ratio (the ratio of theoretical volume of air required for complete combustion per unit volume of methane and n-heptane blended fuel to the actual volume of air supplied) was between 0.8 and 1.2, for which methane content was varied from 0% to 50%. The experimental results showed that with the increasing methane content the main unburned hydrocarbons (C2H2, C3H8, C2H4, and C4H10) and CO2 decreased, while CO increased. The sensitivity analysis showed that R24 (CO + OH = CO2 + H) plays a key role in the emissions of unburned hydrocarbon and carbon monoxide. At the same C/O ratio of n-heptane/air and n-heptane/methane/air mixtures, the reaction rate of R24 increased with the addition of methane, which in turn reduced the hydrocarbon and carbon monoxide emissions and improved the combustion efficiency.