In this study, the lubrication characteristics of sliding members were compared with the changes in the hardness of friction surfaces and the application of nano-oil. The materials of the specimens were gray cast iron (AISI 35 and AISI 60) and nickel chromium molybdenum steel (AISI 4320). The friction coefficients and the temperature variations of the frictional surfaces were measured with a disk-on-disk tribotester under a fixed rotation speed. The friction surfaces were observed with a scanning electron microscope (SEM). The friction coefficients of the plate surface increased as the hardness difference increased. The friction coefficient after the lubrication with nano-oil was less than that after lubrication with mineral oil. This is because a spherical nanoparticle plays the role of a tiny ball bearing between the frictional surfaces that improve the lubrication characteristics.