This paper describes a simulation guided design methodology for developing direct injection combustion systems of gasoline engines. The first step is the optimization of engine gas flow. The intake port is optimized by CFD simulations to compromise the engine breathing capacity and its tumble flow. Secondly, the piston crown shapes and the injection system designs (injection pressure, hole number, hole size and orientations) are optimized based on dedicated CFD simulation results. Thirdly, different injection strategies are used at different engine operating conditions to achieve best engine performance, such as split injections being used at cold starting and catalyst heating period to realize stratified charge combustion for fast catalyst light-off, and a single injection being used to achieve homogeneous mixture combustion at almost all other operating conditions. This methodology was applied to develop a gasoline direct injection system to replace a MPI fuel system of a 1.6 liter naturally aspirated engine. The dyno data of the developed engine show that, comparing with the baseline MPI engines, the developed combustion system can increase the engine torque by 5% ∼ 23%, the maximum power by 4.5%, and decrease the fuel consumption under vehicle NEDC drive cycle by 3.9% (with the original gearbox). And after an optimization design of the transmission, the fuel consumption under NEDC working condition improves by 6.6%.