Multiple diesel engine systems combined with several medium-speed engines have been recently developed as a propulsion system for ocean-going vessels. If bunker oil can be used satisfactorily as a fuel for these medium-speed engines, the operation cost of ships would be more reduced. However, the test of bunker oil as a fuel heated up to high temperature has not been examined yet for this type diesel engine by this time. This paper describes experiments to investigate the effect of fuel temperature on combustion for diesel engines. The results present that the fuel consumption and the exhaust temperature of the engine are increased with raising fuel temperature up, on the contrary of the authors expectation. An analysis is also made of these results.