An experimental investigation was conducted on a spark ignition engine using Gasoline and CNG . The aim is to compare the effects of both fuels on the steady and transient performance of the engine as well as the exhaust emission characteristics . A complete facility for the testing was constructed and operated. The test rig includes, besides the 1.301 litre spark ignition engine and the Heenan hydraulic type dynamometer, all instrumentation necessary for evaluating engine steady and transient performance, and exhaust pollution characteristics. The test rig was also furnished with computer controlled data acquisition systems. It was concluded that using CNG increases the brake specific fuel consumption and decreases the volumetric efficiency, brake thermal efficiency, and exhaust gas temperature. It also increases CO concentration in the exhaust gases at full loads but slightly decreases it at part loads. The propane and methane hydrocarbon emissions are increased with CNG at all engine operating conditions . The mixture equivalence ratio studied has the greatest influence on engine performance and emission. The transient acceleration test results showed that using CNG decreases engine response to a sudden acceleration than using gasoline. The transient load acceptance test results showed that using CNG decreases the ability to overcome the sudden load than gasoline.