Since in-cylinder flame temperature has a direct effect on an engine's NOx characteristics, this phenomena has been studied in detail in a multi-cylinder DI diesel engine using a new method allowing the in-cylider temperature distribution to be measured by the two color method. An endoscope is installed in the combustion chamber and flame light introduced from the endoscope 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 and KL factor are immediately calculated by a computer using the two color images from the CCD camera.In the case of EGR, the test was conducted under 75% load conditions. The flame temperature was reduced according to an increase of EGR rate. Formerly the main reason for NO reduction with EGR was considered to be the reduction of oxygen concentration, however the flame temperature was considerably reduced with EGR in this test. Therefore the rates of contribution to NO reduction was estimated using a modified KIVA-II code. From this result, the contribution rates to NO reduction are nearly the same for both oxygen concentration and flame temperature.