A 2-Stage boosting heavy-duty diesel engine made up of a relatively small turbocharger and a conventional turbocharger was simulated and experimented in order to improve the fuel consumption, especially in a low engine speed region where most of heavy-duty diesel engines are frequently operated. In the low engine speed and high load region, this system obtained high boost pressure. As the result, fuel consumption improvement and BMEP increase were simultaneously achieved in the low engine speed region. In other regions, the control of boost pressure and PMEP is very important so as to maintain good fuel consumption and low exhaust emissions.