Effects of concavities formed by piston skirt profile or pattern coating on the piston skirt on the friction reduction in an internal combustion engine were investigated by calculation. Calculations of oil film were carried out under assumption of insufficient oil quantity at the concave based on experimental results. Friction loss increased when there was no oil in the concave. It was found that when there was oil in a part of the concave, depending on the concave types, the friction loss was lower compared with the condition where the concave was completely filled with oil.