The strict regulation of environmental laws, the oil price and restricted resources has made the vehicle manufacturers to use other energy resources instead of fuel oil. Iran is recognized as the second holder of gas reservoirs in the world and can use hydrocarbon gases broadly in particular compressed natural gas (CNG) as the fuel for vehicles specifically in its public transportation fleet and thereby reduce the consumption of diesel fuel and gasoline. This will bring about the reduction of environmental pollutants and reduce the economic costs of transportation sector. With regard to the climatic situation of Iran and concerning the existence of broad network of gas distribution, CNG is a suitable alternative for other fuels. Therefore, developing bi-fuel engine (gasoline and CNG) in the short and middle term strategy for achieving this important subject will be necessary. A basic measure for supporting the subject is applied studies for considering and improving the engine performance. In this paper, a four-stroke bi-fuel spark ignition (SI) engine has been modeled. The model is based on the two-zone combustion model. The selective outputs are such as volumetric efficiency, brake power (BP), brake mean effective pressure (BMEP), torque, brake specific fuel consumption (BSFC) and emissions. In this study, the effect of engine speed, equivalence ratio and performance parameters have been discussed and considered. In addition, the model has been validated by experimental data of an engine performance result. The CNG with regard to the gaseous form specification has advantages and disadvantages as compared with gasoline. The natural gas forms a more homogenous mixture in comparison with gasoline. It is cheaper than gasoline and produces the least rate of CO while gasoline produces more power and less NOx as compared with CNG. In order to obtain an engine with less pollution and better performance, it should be designed for each type of fuels specifically.