The objective of this paper is to investigate the potential of lean burn combustion to improve the thermal efficiency of spark ignition engine. Experiments used a single cylinder gasoline spark ignition engine fueled with primary reference fuel of octane number 90, running at 4000 revolution per minute and at wide open throttle. Experiments were conducted at constant fueling rate and in order to lean the mixture, more air is introduced by boosted pressure from stoichiometric mixture to lean limit while maintaining the high output engine torque as possible. Experimental results show that the highest thermal efficiency is obtained at excess air ratio of 1.3 combined with absolute boosted pressure of 117 kPa. Three dimensional computational fluid dynamic simulation with detailed chemical reactions was conducted and compared with results obtained from experiments as based points. The potential to improve further the efficiency, exhaust gas recirculation (EGR), high engine swirl ratio and high research octane number are good candidates for this improvement.In further calculations, we use simulated EGR ratio, high engine swirl ratio, RON95 fuels. To prevent the combustion from knock in the simulations when ignition timing is at maximum brake torque, simulated pressure probes are mounted on each side of intake and exhaust valves to detect knock pressure. By using these methods, spark timing of the engine can be advanced thus improve the thermal efficiency. It is found that, by combining lean-boosted/EGR and high octane-number fuel, higher thermal efficiency of spark ignition engine can be achieved.