In order to obtain some information on ignition property of diesel fuels, we have attempted to determine autoignition temperatures of various diesel fuels and to compare those with their cetane numbers. As a result it was shown that the autoignition temperature of the diesel fuels should have a tendency to decrease with an increase in their cetane number and that the addition of the cetane number improving agents could make autoignition temperatures lower. We suggest that the autoignition temperature test should be useful screening one to evaluate the cetane number and the cetane number improvement of diesel fuels.