A piston ring which can sustain a superior effect of preventing aluminum cohesion for a long time in a high output engine is provided. At least one of the upper and lower side faces of a piston ring is coated with a polyimide film having hard particles dispersed therein. Employed as the hard particles may be alumina, zirconia, silicon carbide, silicon nitride, cubic boron nitride, or diamond. The hard particles are to be 0.01 to 5 μm in average particle diameter, and the film is 2 to 30 μm in thickness.