This paper presents an analytical modeling of piston secondary motion for a small gasoline engine with the objective of reducing piston slap excitation. To analyze the vibration, a modeling of the reciprocity machinery iscarried out and numerical simulation is performed. In this model, both dynamic characteristics of the part of piston skirt and cylinder wall, which are obtained from FEM model, are taken into account and experimental study is also conducted to verify the analytical results. As compared the simulated results with the experimental ones, the validity of presented model has been confirmed and this numerical model is effective to comprehend the piston slap secondary motion.