Autoignition and combustion characteristics of biomass gas were investigated experimentally. A mock biomass gas consisting of H2, CH4, CO, N2 and CO2 was used as a wood pyrolysis gas. Experiments were carried out with a modified gas engine varying equivalence ratio and fuel composition. High hydrogen content increases the combustion speed, but it hardly affects ignition timing. Carbon monoxide in fuels does not affect combustion speed largely. The autoignition temperature of biomass gas is about 1000K, which is the same as those of hydrocarbon fuels. The engine also realizes 42% of the indicated thermal efficiency and a maximum IMEP of 0.3MPa.