An experimental investigation on the influence of the excess air ratio and hydrogen fraction on the emissions characteristics of a spark-ignition engine fueled with natural gas - hydrogen blends was conducted. The results indicate that the excess air ratio has a significant effect on the hydrocarbon (HC), NOx, CO, and CO2 concentration for both pure natural gas and natural gas - hydrogen blends. For a specified excess air ratio, HC emissions decrease with the increase of hydrogen fraction; the behavior is more obvious under the lean burn operation. The NOx concentration increases with the increase of hydrogen fraction, and NOx gets its peak value at an excess air ratio of 1.1. CO2 emissions decrease with increasing hydrogen fraction. Meanwhile, the addition of hydrogen into natural gas can extend the lean burn limit of a mixture. Thus, an engine fueled with natural gas - hydrogen blends operating under lean mixture conditions can get low emissions of HC, CO, CO2, and NOx.