One method is the pilot injection in order to clear up the urgent subjects imposed on a DI diesel engine for the exhaust emissions without falling its high thermal efficiency. The pilot injection method can be expected to bring forth a shorter ignition delay, a low peak pressure and a low pressure rise, that is, a slow heat release. The possibility to apply the pilot injection to a DI diesel engine is discussed in this paper. The effectiveness of the pilot injection is estimated by the temperature and the heat release rate calculated from the pressure histories in cylinder.The followings are obtained; (1) The suitable timing of the pilot injection can shorten the ignition delay of the main fuel, and reduce the maximum pressure and the higher heat release at earlier combustion stage. (2) The minimum quantity of pilot injection is 8-10 of the excess air ratio or 0.7-1.0 % of the weight ratio of the air in cylinder. (3) The suppression of NOx emissions can be expected without the worsening of the smoke and HC emission.