Since in-cylinder flame temperature has a direct effect on an engine's NOx characteristics, these phenomena have been studied in detail in a DI diesel engine using a newly developed method allowing the in-cylinder temperature distribution to be measured by the two color method. The flame light introduced from the visualized combustion chamber of the engine is divided into two colors by filters. The images of combustion phenomena using the two wavelengths are recorded with a framing streak camera which includes a CCD camera. The flame temperature is immediately calculated by a computer using two color images from the CCD camera.A parameter study was then carried out to determine the influence of intake valve number of the engine, and fuel injection rate (pilot injection) on the in-cylinder temperature distribution. In the case of one intake valve (2-valve engine), the mean flame temperature within the observation region was about the same level as the two intake valve (4-valve engine), whereas the maximum temperature was higher than that of the two intake valve engine. In the case of pilot injection, the mean flame temperature was slightly reduced compared to that of the conventional injection.It is considered that a lower flame temperature causes a low level of NOx emission. Therefore, a reasonable result was obtained from the flame temperature measurement.