This paper describes our investigations of the effects of split injection on the combustion and emission characteristics of a D.I. Diesel engine with a common-rail injection system. The ratio of amount of fuel injected between two consecutive injection pulses was varied while the total injection period was kept constant. As compared with single injection, having an injection period shorter than that of split injection, the split injection showed lower NO_xand higher smoke emissions and inferior brake specific fuel consumption (BSFC) . Increasing the ratio of amount of the first injection improves BSFC, while maintaining the reduction in NO_xemissions from single injection. Test results indicate a possibility that split injection could reduce NO_x emissions without an increase in BSFC and with only a slight increase in smoke emissions.