For reduction of NOx and soot emission with conventional diesel diffusion combustion, the authors focused on enhancement of the rate of injection (hereafter referred to as RoI) to improve air availability, thus enhancing the fuel distribution and atomization. In order to increase opening ramp of the RoI (hereafter referred to as fast injection rate), a hydraulic circuit was improved and nozzle geometries were optimized to make the greatest use of the advantages of the hydraulic circuit.Two different common rail injectors were prepared for this research. One is a mass production-type injector with piezo actuator that achieved the EURO-V exhaust gas emission standards, and the other is a prototype injector equipped with the new hydraulic circuit. The nozzle needle of the prototype injector is directly actuated by high-pressure fuel from common rail to improve the RoI. As a result, the prototype injector used for this research has a property as approximately 80% faster injection rate than the mass production-type.The liquid phase (non-vapor) spray characteristics were evaluated using a high-pressure vessel, high-speed video camera, and metal halide lamps as the light source. In addition, the spray vapor characteristics under the same temperature and pressure conditions as an actual engine combustion chamber were evaluated with the high-temperature, high-pressure vessel using the LIF method. The combustion characteristics were evaluated using a single-cylinder engine system with same specification of the four-cylinder mass production diesel engine. At the set point of the EURO-V NOx level under the 1500 rpm half-load operating conditions, soot was reduced 48% without deterioration in combustion noise. This new prototype injector has the performance of liquid phase spray penetration that is 23% longer than the mass production-type, and the spray area is expanded and average light intensity number of spray using LIF method is 31% lower than mass production-type at the starting point of combustion, 0.5 ms after the start of injection. It was confirmed that faster injection rate causes a longer liquid phase spray penetration, improves fuel distribution and atomization itself, thus reducing soot greatly.