An experimental investigation on the effect of hydrogen fraction on the combustion characteristics of a spark ignition engine fueled with low heat value gas-hydrogen blends was studied. The results show that engine indicated thermal efficiency, indicated mean effective pressure and maximum combustion pressure are increased with the increase of hydrogen fraction in the blends. And hydrogen addition shows remarkable influence on engine power and emissions. At the same excess air ratio, HC emissions decrease, CO and NOx emissions increase with the increase of hydrogen fraction in the blends. And engine power is influenced by both hydrogen fraction and heat value in low heat value gas-hydrogen blends combining. Hydrogen significant extends the lean burn limit of combustion of low heat value gas.