In order to make clear the effect of the cooling methods and of the flow quantity of oil on the piston temperature, the authors have measured some temperature distributions over the cast iron piston of a small engine, with some thermocouples. As the result of a series of experiments, the following conclusions have been reached : 1) In case of the jet cooling, there is a critical oil flow beyond which even if we may increase the quantity of oil flow we can not expect the decrease of the piston temperature. It is 1.5∼2.0 l/cm2/h with respect to the surface area of the piston. (For higher speed, greater quantity required.) 2) At the critical flow, the mean coefficient of heat transfer from the inside of the piston crown to oil is 1600∼1700 kcal/m2h°C. 3) In utilizing the oil jet from the nozzle located at the small-end of the connecting-rod, the mean coefficient of heat transfer approaches to the maximum local coefficient.