Nowadays increasing emission regulations and environmental concerns have led to the use of alternative fuels in internal combustion engines. Biogas is an alternative fuel that can be produced with clean, green and natural resources. In this study, a single cylinder, four stroke SI engine was operated at stoichiometric conditions, 2800 rpm constant engine speed with biogas fuel at different ratios. As biogas fuel, methane (CH4) and CO2 were used during engine tests. Test results which were taken by using only gasoline were compared with the results obtained by using biogas fuel at different CO2 ratios (10%, 20%, 30% and 40% by volume). Although it was determined specific CO emissions decrease and THC emissions slightly increases with the increasing CO2 ratio, it is foreseen that the increase in THC emissions can be prevented by the use of three way catalytic converter. Although the NOx emissions increase with the use of biogas compared to gasoline, it was significantly reduced by the increased CO2 rate in the biogas fuel, the CO2's heat carrying capacity and reducing the O2 availability of the fuel. On the other hand, because of the high cost of purification process of biogas and after-treatment equipment, it is foreseen that use of biogas at high ratio of CO2 in SI engines will be advantageous in terms of NOx emissions.