Power boosting technology of a High Speed Direct Injection (HSDI) Diesel engine without increasing the engine size has been developed along with the evolution of a fuel injection system and turbocharger. Most of the turbochargers used on HSDI Diesel engines have been a wastegated type. Recently, the Variable Geometry Turbocharger (VGT) with adjustable nozzle vanes is increasingly used, especially for a passenger car in European market. This study describes the first part of the experimental investigation that has been undertaken on the use of VGT, in order to improve full load performance of a prototype 2.5 liter DI Diesel engine, equipped with a common rail system and 4 valves per cylinder. The full load performance result with VGT is compared with the case of a mechanically controlled wastegated turbocharger, so that the potential for a higher Brake Mean Effective Pressure (BMEP) is confirmed. Within the same limitation of a maximum cylinder pressure and exhaust smoke level, the low speed torque could be enhanced by about 44% at maximum.