Chemical modification of diamond surface was performed using benzoyl peroxide with CN group containing compound. When acetonitrile was used as the solvent for a radical reaction, the CN group was introduced on the diamond surface. On the other hand, when benzonitrile was used as the solvent for a radical reaction, no CN group was introduced on the diamond surface. Acetonitrile has an alkyl group in the structure. The radical species derived from benzoyl peroxide must abstract the hydrogen atom in the alkyl group, and then the radical species generated from acetonitrile should react with the diamond surface. Bensonitrile has no alkyl group in the structure. No hydrogen abstraction reaction, therefore, must proceed in the reaction process. This should be the reason for the nonintroduction of the CN group on the diamond surface.