We investigate the effects of etching gases on the synthesis of nano crystalline diamonds grown on silicon substrate at the substrate temperature of 550 • C and the reaction pressure of 4 kPa by hot filament chemical vapor deposition method, in which CH 4 and H 2 act as a source and diluting gases, respectively. N 2 , H 2 , and NH 3 were used as the etching gases, respectively. Results show that the optimum conditions can be obtained only for the case of H 2 gas. The crystal morphology and crystallinity of the samples have been examined by scanning electron microscopy and X-ray diffraction, respectively.