Abstract: Nowadays, increased attention has been focused on internal combustion engine fuels. Regarding environmental effects of internal combustion engines particularly as sources of pollution and depletion of fossil fuels, compressed natural gas has been introduced as an alternative to gasoline and diesel fuels in many applications. A high research octane number which allows combustion at higher compression ratios without knocking phenomenon and with good emission characteristics of unburned hydrocarbons and carbon monoxide are major benefits of compressed natural gas as an engine fuel. In this paper, natural gas as an alternative fuel in a spark ignition engine, has been considered. Engine performance and exhaust emissions have been experimentally studied for both natural gas and gasoline fuels in a wide range of engine operating conditions.