The effect of incorporating oxygen and nitrogen into the feed gases on the texture and surface morphology of diamond film synthesized by hot filament chemical vapor deposition (HFCVD) is investigated. The reactant gas composition is determined by the gas flow rates. At a constant flow rate of hydrogen (33 seem) and methane (0.68 seem), the oxygen and nitrogen were varied in the O/(O + C) ratio from 0.05 to 0.43 and in the N/(N + C) ratio from 0.15 to 0.60. The films were grown under a constant pressure (20 Torr) and a constant substrate temperature (800 degrees C). Clearly nitrogen in the reactant gases has a distinct tendency to promote the [100] texture and the corresponding {100} morphology, whereas oxygen promotes the development of [111] texture and {111} morphology. According to the Wulff theorem (Gamma = d(100)/d(111) = gamma(100)/gamma(111)) and the evolutionary selection of crystallites and the surface configurations of diamond, the results reveal that during growth nitrogen plays a critical role in activating the CD-H surface site and consequently increases the surface free energy gamma(111), Of the {111} surface. In contrast, oxygen activates the C-D=H-2 surface site and increases the surface free energy gamma(100), of the {100} surface. These results indicate that the texture and the surface morphology of polycrystalline diamond film can be completely controlled by the reactant gas composition.