In recent years, the importance of engine performance at part load has gradually increased in marine generator engines because they are generally operated at part load. So, various studies were carried out to reduce fuel oil consumption at part load for a medium speed diesel engine, HiMSEN Engine, which is generally used as a marine generator, in order to satisfy the needs of customers. The basic concept for the improvement of fuel oil consumption at part load is increasing maximum cylinder pressure and fuel injection pressure. In this study, the air waste-gate system is introduced and evaluated through the performance simulation. The main performance parameters of the combustion system such as fuel cam shape, fuel injection nozzle, turbocharger specification and engine compression ratio are also evaluated and optimized by using performance and combustion simulations. Finally, the simulation results show that the specific fuel oil consumption at 85% load can be reduced by 2.9% with the combination of these methods.