Convective heat transfer of an impinging jet is numerically evaluated for piston cooling process. A circular jet of subcooled engine oil that impinges normally onto the inner surface of the piston for an engine operating at normal condition is considered in the study. The shear stress transport (SST) based on transient three-dimensional governing Navier–Stokes (Reynolds-averaged Navier–Stokes (RANS)) equations are computationally solved using a finite-volume technique. The conjugate heat transfer method is used to obtain a coupled heat transfer solution between the solid and fluid regions, to predict the heat transfer coefficient at the piston walls and then the temperature distribution in the piston. It is shown that the cooling jet can significantly decrease the piston temperature. The location of the incidence of maximum heat transfer coefficient is moved away from the impingement point as the nozzle size increases.