The effects of surface roughness on the friction characteristics of FC250 cast iron coated with manganese phosphate was examined by using a ring-on-disk test machine. A rotating test piece and a stationary one were used under test conditions, with only the rotating piece coated. The test piece surfaces either had a roughness (Rz) of 0.8, 3.2 or 6.3μm, and the contact pressure at which the pieces were pressed to each other was gradually increased from 1MPa to 9MPa. A manganese phosphate coating is easily worn out, and then a part of the base material is exposed. This makes the surface's friction stable. It was shown clearly that the frictional force strongly depends on the load as well as on the base material's surface roughness. The friction coefficient was as high as 0.04-0.05 when both pieces had the same roughness, but was as low as 0.007 when the pieces had different roughnesses.