Effects of pilot injection on diesel combustion have been studied on a turbocharged direct-injection diesel engine. Under various engine operating injection conditions, emissions were measured while the pilot quantity and timing were varied. The result showed that the pilot injection at low engine load could reduce NOx and THC and, also, improve fuel consumption in some degree. To grasp the phenomena, diesel combustion was analyzed and combustion process observed with an endoscope. It was found that the pilot injection reduced average combustion gas temperature due to a restriction on pre-mixed combustion and a slower combustion during diffusion. The photographs of the entrainment of the burned gas, generated from the pilot combustion, by the main fuel spray injected into the pilot flame and of the resulting slow down of the diesel combustion were taken.