In this paper, the potential of combined injection of CNG and gasoline is studied on a 1.7 L turbocharged, port-injected SI engine and the best engine performance point for the best conversion efficiency of the catalytic converters has been investigated. Compressed natural gas (CNG) as an alternative fuel is used in spark ignition engines to improve fuel consumption and exhaust emissions. The improvements gave more advantage in emission but it lowered the performance of the engine. As a substitute, CNG has a higher octane number and knocking resistance than gasoline and hence CNG-dedicated engines can have higher compression ratios and therefore higher indicated efficiencies. Turbocharged bi-fuel, combined CNG and gasoline, injection engine of is a new concept which offers direct benefits with regards to gas or gasoline powered vehicles running separately on each fuels. It also opens very interesting perspectives for meeting future emission regulations using only a three-way catalyst, since the stoichiometry condition of combustion is maintained over the whole engine operating range. Results show that the combined injection of gasoline and CNG is much better than gasoline mode in terms of fuel consumption and raw HC and CO emissions. However, as expected the NO x emission will increase. According to the obtained results at 16.2 bar BMEP, 3000 rpm full load condition with 30% CNG mass fraction, the BSFC, CO and HC emissions are improved by 16, 66 and 50%, respectively, compared to gasoline single mode. It was found that a fuel mixture of 30% CNG mass fraction was the best trade-off point between engine performance and emission production. Also, significant reductions of fuel consumption were observed. Full-load tests carried out with a turbocharged engine enhanced the synergy effect between the two fuels at full-load condition.