This study presents the numerical simulations of combustion in a spark ignition engine fueled with biogas (65% CH4-35% CO2) through a cylinder. The numerical calculations were performed using the finite volume CFD code FLUENT with the RNG k-epsilon model, for which different compression ratio (CR) of 8.5, 10.0, 11.5 and constant nominal speed of 1500 rpm respectively. Excess air ratio (EAR) values were selected as 1, 1.1, 1.2, 1.3 and 1.4. The spark timings were started from 19 degrees BTDC. The results of the combustion process were investigated as a function of crank angle. Numerical cylinder pressure data was compared with experimental data for the verification of the combustion model, this investigate reveals that the combustion model used shows encouraging results. When the excess air ratio increased, peak pressure and temperature values are decreased. Peak pressure and temperature values are obtained at CR = 11.5 and EAR = 1.1. At biogas combustion, higher compression ratio should be preferred.