Abstract: Oil-cooling was proposed and tested in a small high speed diesel engine of direct injection. Both steady and instantaneous wall temperatures of combustion chamber were measured under various working conditions. It was recorded that the wall temperature was considerably elevated by means of oil as the coolant. The engine fuel consumption was improved, particularly at part load condition, with oil-cooling in comparison with water-cooling on the same engine. So was the cold starting performance. The improvement is attributed to the reduction of heat rejection from the increase in wall temperature.