Abstract: The pressure fields and friction forces at the skirt of the piston were analyzed by using relevant mathematic models based on hydrodynamic lubrication theory and piston dynamics. Thus the transverse movement of the piston was analyzed and the pressure field and friction forces at the skirts of the piston were calculated by taking into account the out-of-roundness of the cylinder liner produced in the running of the gasoline engine. The results indicated that the peak value of the pressure field of the oil film decreased and the friction forces at the piston skirt increased after considering the out-of-roundness of the cylinder liner. The larger the degree of the out-of-roundness of the cylinder liner, the larger the increments of the friction force and friction power consumption. Thus it was imperative to take into account the effect of the cylinder liner out-of-roundness on the lubrication and friction characteristics of the piston skirt in the design of the piston.