The wear resistance of the Mg-treated nodular irons having various matrix structures under sliding contact to the rotating discs made of grey iron or Ni-Cr steel was measured. The sliding velocities were 4.2m/sec and 2.5m/sec, and the contact pressure was 8.3kg/cm2. When worn against pearlitic iron having flaky graphite, the wear amount of nodular iron is smaller than that of flaky iron. If worn against steel, it is difficult to find difference in the wear resistance between nodular and flaky iron. It is presumed that the self-lubricating property of flaky iron is larger than that of nodular iron.