NO and Soot trade off is an important challenge for engineers in DI Diesel engines. This paper, introduces multiple injection as a strategy for simultaneous reduction of NO and Soot emissions on a DI diesel engine and also proposes a new concept of variable injection pressure and studies its effect on the engine emissions. To evaluate the benefits of multiple injection strategies and to reveal combustion mechanism, modified three dimensional CFD code KIVA-3V was developed. Results showed that using post injection with appropriate dwell between injection pulses has a great effect on simultaneous reduction of the emissions. Based on computational results, NO reduction formation mechanism in multiple injection strategy is as a single injection with retarded injection timing. It is shown that reduction in soot formation is because of the fact that in split fuel injection the soot, which is producing rich regions at the head of fuel spray, are not replenished by newly injected fuel in post injection pulse. Also increasing injection pressure in post injection will reduce the Soot emission dramatically while NO emission is in control for increase of fuel burning rate in post injection pulse.