Diamond deposition on pure titanium and iron substrates pretreated in N-2-C2H2 plasma without any mechanical surface pretreatment was studied. High quality diamond film was deposited on such pretreated substrates by CVD using combustion flame. After the pretreatment, a C-C bond was detected on the modified substrate, and graphitic carbon formation on the substrate was suppressed during diamond deposition. The surface of the modified substrate also became slightly rough, so the anchor effect between the film and the substrate was expected. This suppression and the anchor effect would indicate a remarkable improvement of adhesion of the film to the substrate. In particular, the diamond deposition process on the plasma-pretreated iron substrate was compared in detail with that on the substrate scratched with diamond powder. It was found that diamond was directly grown from the carbonitrided surface layer of the plasma pretreated substrate.