An experimental study on the performance and emissions of a spark ignition engine operating on the natural gas-hydrogen mixtures was conducted. The results show that the engine lean-burn limit is extended by the addition of hydrogen into natural gas. For a specific excessive air ratio, engine power output and thermal efficiency decrease with the increase of hydrogen fraction in natural gas when the hydrogen fraction is less than a certain value (20%) whereas engine power output and thermal efficiency increase with further increasing hydrogen fraction when hydrogen fraction is larger than a certain value (20%). Addition of hydrogen into natural gas decreases the exhaust hydrocarbon (HC) concentrations. However, addition of hydrogen into natural gas will increase NOx concentration. Thus, an engine operating on lean-burn natural gas-hydrogen combustion is favorable for getting higher thermal efficiency and lower emissions.