Combustion behaviors of a direct injection engine operating on various fractions of natural gas-hydrogen blends were investigated. The results showed that the brake effective thermal efficiency increased with the increase of hydrogen fraction at low and medium engine loads and high thermal efficiency was maintained at the high engine load. The phase of the heat release curve advanced with the increase of hydrogen fraction in the blends. The rapid combustion duration decreased and the heat release rate increased with the increase of hydrogen fraction in the blends. This phenomenon was more obviously at the low engine speed, suggesting that the effect of hydrogen addition on the enhancement of burning velocity plays more important role at relatively low cylinder air motion. The maximum mean gas temperature and the maximum rate of pressure rise increased remarkably when the hydrogen volumetric fraction exceeds 20% as the burning velocity increases exponentially with the increase of hydrogen fraction in fuel blends. Exhaust HC and CO2 concentrations decreased with the increase of the hydrogen fraction in fuel blends. Exhaust NOx concentration increased with the increase of hydrogen fraction at high engine load. The study suggested that the optimum hydrogen volumetric fraction in natural gas-hydrogen blends is around 20% to get the compromise in both engine performance and emissions.