A diamond stone is worn away faster than a cBN stone when superfinishing a steel workpiece, because of the carbon diffusion into iron. However, if such a wear mechanism can hardly work, it is expected that the wear of a diamond stone decreases. This paper describes two methods to suppress the carbon diffusion; that is, nitriding process of steels and addition of reactive BaSO_4 abrasive to a diamond stone. When nitriding is applied to a steel, the face-centered cubic γ' nitride, Fe_4N, is formed at the steel surface. It acts as a barrier to deter the diffusion of carbon into iron. On the other hand, BaSO_4 abrasive added to a diamond stone forms an oxide layer of Fe_3O_4 on the steel surface during superfinishing process. The formation of Fe_3O_4 accelerates the deoxidization by diamond. Then, the diamond stone wears not by carbon diffusion but deoxidization of Fe_3O_4. Superfinishing experiments have showed that both these methods improve the wear resistance of diamond stone.