In this study, the effect of fuel type; gasoline or natural gas on the energy and exergy balance as well as the performance of spark ignition engine was experimentally investigated. The experiments were conducted using a four cylinder, naturally aspirated CNG-gasoline bi-fuel engine at wide open throttle (WOT) operating condition. The results showed that when engine was fed with gasoline, the output power was higher than that of gaseous fuel by 4.2 kW on average throughout the engine speed range. Thermal efficiency of the engine with natural gas was higher than that of the gasoline by approximately 5.4% throughout the engine speed range. In addition, CNG fuel showed higher exergetic efficiency than gasoline, and based on these results second law efficiency of CNG engine was higher than that of gasoline engine by 3.18% on average. This was largely due to combustion temperature increase in CNG case. For all operating points, the percentage of energy and exergy transfer through exhaust gases and the cooling system in gasoline are lower than CNG. However the destructed exergy of gasoline was higher than CNG by about 5.8%, averagely. (C) 2015 Elsevier Ltd. All rights reserved.