The purpose of this paper is to investigate the effect of the piston shape and weight on the friction losses. The motoring test was conducted on pistons with various shapes, rigidities, and weights at 3000-16000 rpm. The test results suggested that the piston friction loss would be abruptly increased mainly because of the increase in the skirt friction caused by the change of the piston attitude. Then, the piston behavioral simulation was conducted using a two-dimensional rigid model in consideration of the skirt rigidity, which proved the above suggestion. This paper consists of the paper presented at 1990 JSAE Autumn Convention and the results of the investigation we conducted based on the calculation of piston behavioral simulation.