There are several physico-chemical processes which affect diesel combustion. Among them the processes occurring prior to the ignition have significant influence on the combustion and emission characteristics of direct injection diesel engine. The premixed phase combustion appearing between the ignition delay period and the mixing controlled combustion phase has been recognized to be important from emission standpoint. The fuel mass fractions burned in the premixed phase is dependent on the mass preparation rate during the delay period. Based on this consideration, a gas jet multizone phenomenological model has been used for the investigation concerning the premixed combustion phase. The results concerning effect of important engine parameters viz. injection timing, injection pressure and engine speed are discussed in this paper. The effects of these parameters on engine emissions are inferred from the mass fractions burned at the ignition.