In this study, based on a 12L class diesel engine, a natural gas-diesel dual fuel engine was developed by adding natural gas fuel supply system. For optimal control of dual fuel engine, a conventional diesel engine ECU and a dual fuel ECU were utilized. To convert the dual fuel engine, MPI natural gas injectors were installed on the new modified intake manifold adapter. As a results, the dual fuel engine showed same level of torque, power performance and exhaust gas emissions as those of a diesel base engine.. Furthermore, overall fuel replacement rate was 70~76 % and total fuel cost saving was 37~40%.