A great deal of attention has been focused on decreasing the piston assembly friction in piston engines, since this friction acounts for the majority of total mechanical power loss in these type of engines. The tribology of the sliding surfaces between piston rings and cylinder liner includes extremely complicated phenomena which will become even more severe with an increase in the engine power. In the present study, the theoretical analysis on the mixed lubrication of piston rings is made. The analysis is comprised of Patir and Cheng's average flow model and Greenwood and Tripp's asperity interaction model, and is developed to consider the shear thinning effect of non-Newtonian fluid, multigrade oil. The friction characteristics of piston rings for both monograde and multigrade oils are investigated and compared with experimental results obtained by the floating liner method, which can measure the instantaneous friction force of a piston assembly under firing conditions.