Abstract: We investigate the effects of etching gases on the synthesis of nano crystalline diamonds grown on silicon substrate at the substrate temperature of 550 oC and the reaction pressure of 4 kPa by hot filament chemical vapor deposition method, in which CH₄ and H₂ act as a source and diluting gases, respectively. N₂, H₂, and NH₃ were used as the etching gases, respectively. Results show that the optimum conditions can be obtained only for the case of H₂ gas. The crystal morphology and crystallinity of the samples have been examined by scanning electron microscopy and X-ray diffraction, respectively.