A well-known effective way to improve thermal efficiency of spark ignition engine is to use hydrogen mixed with gasoline. Hydrogen, even in small amounts, it makes flame speed faster, knock mitigation and improves combustion stability. These benefits enable higher compression ratio, higher exhaust gas recirculation(EGR) rates and improved engine thermal efficiency. Fuel reforming is one way to get hydrogen from the internal combustion engine, using hydrocarbon fuel, exhaust gas steam and heat, hydrogen reformation is achieved using species already available in the exhaust gas. However, chemical reaction is restricted by light-off temperature of reforming catalyst, so fuel reforming is not available at low speed and low load engine operating condition due to low temperature of EGR gas to heat up catalyst. Thus, the objective of this research is to enlarge available reforming zone to low speed/low load engine operating condition by optimization of fuel reforming system using waste heat recovery and to improve fuel economy on a 4 cylinder turbo gasoline direct injection(TGDI) engine. It is verified that the combustion speed is increased, EGR limit is expanded and knock is reduced when hydrogen from On-Board fuel reforming system is supplied into combustion chamber. It is found that more hydrogen is generated under higher catalyst temperature, and combustion speed is faster by more hydrogen. It is possible to increase catalyst temperature by newly designed fuel reforming system using exhaust heat recovery, and it enabled to enlarge reforming available zone into low speed and low load operating condition. It is confirmed that on-board fuel reforming system with exhaust heat recovery is a practical way to improve thermal efficiency of TGDI engine.