Silicon carbon nitride (SiCN) is a candidate as underlayer materials for the growth of diamond thin films for hard coating. Diamond thin films were grown on SiCN layers deposited on Si substrates and directly on the Si substrates for comparison, to investigate the availability of the SiCN underlayers. Both diamond films and SiCN underlayers were deposited by hot filament chemical vapor deposition (HF-CVD). The source gas for the SiCN films deposition was hexamethyldisilazane represented as (CH3)(3)SiNHSi(CH3)(3). The number density of diamond crystallites grown on the SiCN underlayers was low as compared with that of the films deposited on the Si substrate. The number density is not affected by the scratch pretreatment of the SiCN underlayers with diamond powder, which is clearly different from that of the films grown on the pretreated Si substrates. This might be because the SiCN films deposited at low temperatures less than 400 degrees C cannot keep the surface morphology formed by the scratch treatment under the high-temperature growth condition of the diamond films. (C) 2016 Elsevier B.V. All rights reserved.