Abstract: This paper presented a novel model with considering the effects of connecting rodcrankshaft variable inertia on the piston secondary motion. The corresponding computing program was developed to predict the piston trajectory and the side force of piston skirt. The results show that the connecting rod inertia variation has considerable effects on the piston secondary motion and the side force of piston skirt, while the effect of the crankshaft variable inertia is weak. The model can be applied to analyze various engine design parameters and operational conditions in the design of internal combustion engines.