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The effects of highly heated fuel on diesel combustion were studied experimentally in a rapid compression machine. A pure fuel, dodecane, heated up to and beyond its critical temperature, was injected into a diesel combustion chamber with the air charge at a compression ratio of 18.2 to 1. The ignition delay was found to decrease with the increase of fuel temperature. The delay decreased to almost zero (within the limit of the accuracy of the instrumentation) at fuel temperatures above 600K. This decrease of delay was explained in terms of a thermal ignition model. For the short ignition delay combustions, the premixed burning phase could not be detected from the heat release data. The mixing controlled burning phases of the heated and unheated fuels however, were not much different; in particular, there was no rapid mixing phenomenon when the fuel temperature was above critical.