An experimental research on combustion instability of a spark ignited single cylinder engine operating on biogas with different distribution ratio of hydrogen (15%, 20%, 25%, 30%, 35% in volume) is studied. The results show that the increase of the hydrogen fraction in the fuel blends leads to the decrease of the cyclic variation coefficient of indicated mean effective pressure in a certain fraction range (15% similar to 35%). The combustion heat release rate accelerates. The flame development angle shortens because the chemical reaction takes the dominant effects. Increasing the ignition advance angle can decrease the cyclic variation. The engine work stability is largely improved with the higher hydrogen fraction in the fuel blends. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.