Abstract: In the constant volume combustion vessel,two-color technique was used to study the effects of different diesel temperatures on the combustion temperature and soot formation characteristics during cold start. The combustion temperature and the KL factor characterizing the soot volume fraction distribution were obtained. The results show that at the combustion stability period of 3.5 ms—6.0 ms after injection,flame at 40 ℃ diesel temperature gives 60% flame area where the temperature is more than 2 200 K in the total flame area,while flame at 20 ℃ diesel temperature and 0 ℃ diesel temperature give 55% and 40% respectively. As the temperature of the diesel decreases,the heat absorption of the diesel fuel increases,and the temperature in the local range decreases,so that the area of the high temperature region of the combustion reduces and the flame temperature is generally reduced. During the combustion stability period,when the diesel temperature is 40 ℃,20 ℃ and 0 ℃,the area with more soot with a KL factor of greater than 0.9 accounts for 60%,50% and 40%,respectively. As the diesel temperature decreases,the ignition delay increases,the oil-gas mixing time increases and the local air-fuel ratio is lager. Therefore,the KL factor decreases as the diesel temperature decreases.