The reduction of NOx without the degradation of soot emission and/or fuel economy necessitates the grasp of the in-cylinder temperature behavior, since NOx is mainly generated within localized high-temperature regions in the cylinder of diesel engines. Therefore, the in-cylinder temperature distributions were measured by the two-color method.This temperature distribution measurement enabled the characterization of the NOx reduction mechanisms for two methods by injection control; injection timing retard and pilot injection.Injection timing retard lowers the temperature throughout the combustion chamber, where NOx is reduced by controlling the spatial development of localized high-temperature regions. On the other hand, pilot injection delays the development of high-temperature regions. NOx is reduced by controlling the temporal development of the localized high-temperature regions.Furthermore, a novel method to reduce NOx has been proposed. This method implies the positive promotion of secondary injection, facilitating the reduction of both NOx and smoke, by abrupt drop in the temperature of localized high-temperature regions around the nozzle hole.