A supercharging system together with a mechanical supercharger and a turbo charger was investigated with experimental and numerical analysis in a diesel engine use for the commercial vehicle. The mechanical loss of a mechanical supercharger was reduced by putting a turbo charger together. Furthermore, it was found that the fuel consumption as a vehicle could be improved by optimizing power train specifications. And it was accomplished by increase of the low-end torque which was produced by a mechanical supercharger.