A combustion model for a spark-ignition engine has been developed in order to study the effect of combustion chamber geometry. The model is based on the two-zone quasi-dimensional analysis. For this model, flame front area is calculated from engine geometry and the spark location. Also, a turbulent entrainment model is used to find burning rate.The model has been calibrated and validated by the experimental data for a pentroof type engine. It has been shown that the model is successful in predicting combustion characteristics at the stoichiometric air/fuel ratio. From this model, the effects of bowl in a piston have been investigated and the optimum bowl size for the combustion is determined. Finally, the effects of the spark plug location, cylinder head type, and piston head shape on combustion process have been examined by the model.