We studied the effects of double injection by a single nozzle on fuel consumption and noise level of a single cylinder 4-stroke direct injection diesel engine. It was observed that, within a certain range of fuel quantity of pilot injection, the combustion noise was reduced considerably. This range of the fuel quantity depends on the timing of the pilot injection. In view of the fuel consumption, it was found desirable to inject pilot fuel at the end of exhaust stroke of the previous cycle or just before the main injection. Furthermore, a thermodynamic analysis was made to explain qualitatively the mechanism of the effects of pilot injection.