The influence of fuel volatility on the thermal efficiency of premixed diesel combustion was evaluated with three ordinary diesel fuels with different distillation temperature distributions and also with a primary reference fuel with an octane number of 20 (PRF20) as a high volatility fuel. The experiments were conducted on a single-cylinder DI diesel engine for the premixed diesel combustion with a single injection at 11% intake oxygen concentration and conventional diesel combustion with a pilot fuel injection at 21% intake oxygen concentration. With the premixed diesel combustion, the indicated thermal efficiencies with the ordinary diesel fuels were lower than with PRF20 although the shapes of the rate of heat release and the combustion efficiencies calculated from the exhaust gas components were almost unchanged. With the conventional diesel combustion, the indicated thermal efficiencies with the ordinary diesel fuels and PRF20 were similar. These results suggest that the deterioration of the thermal efficiency in the premixed diesel combustion with the ordinary diesel fuel is mainly due to the adhesion of high distillation temperature components on the walls of the combustion chamber mainly during the premixing period (between the end of fuel injection and the ignition), and which do not contribute to the effective heat release. The indicated thermal efficiency in the premixed diesel combustion with PRF20 reaches 48%, which is much higher than in the conventional diesel combustion. With a low coolant temperature or a low intake gas temperature at an early injection timing of premixed diesel combustion, the indicated thermal efficiency deteriorates significantly with ordinary diesel fuel while the deterioration with PRF20 is much smaller.