This paper shows the technical trends and future aspects of piston rings for 4-stroke and 2-stroke marine diesel engines. The combination of cast iron and chromium plating is the current standard specification of piston ring for 4-stroke diesel engine. On the other hand, for 2-stroke diesel engine, the combination of cast iron and thermal spray coating, which is applied in accordance with the liner type, is also adopted. In addition, differences in piston ring specification for marine diesel, for example base materials, surface coatings or shape of cross section compared to the small bore 4 stroke diesel engines for trucks, buses and off-high way application are shown. From the viewpoint of technical development, it is assumed that the combination of steel material and PVD coating, likewise heavy duty trucks, will be applied to piston ring for 4-stroke marine diesel engines in order to achieve higher durability in the near future.