The temperature difference between inner and outer races of a roller bearing becomes larger as load and rotating speed of bearing increase, and the effect of rotating speed on the temperature difference seems to be more remarkable than that of load within the allowable carrying capacity of the bearing. Then the effect of rotating speed on the temperature difference between both races has been investigated more in detail. As the result of this investigation, it is found that the temperature difference increases almost linearly with increasing rotating speed under such conditions of experiment, and that the variation of temperature difference in the different kinds of bearings or lubricants is very few. The values of the temperature difference which occur in the moderate running conditions are about 5°C, and this value of temperature difference, which has heretofore been believed generally, has been proved by this experimental research.