The morphology and structure of ultrananocrystalline diamond grown on Si substrate by chemical vapor deposition are systematically studied by varying the methane concentration in the Ar/H-2/CH4 precursor mixture. The films, analyzed by field-emission-gun scanning electron microscopy (FEG-SEM), showed a nanocrystallite cluster morphology that decreased its grain size (L) as a function of the CH4 addition in the gas phase. From Raman spectra, the induced stress of nanodiamond films was analyzed by the G peak shift associated with the defect incorporation and sp(2) bond enhancement. The compressive stress increased as a function of CH4 addition, which also promoted the broadening of the transpolyacetylene peak as indicative of film amorphitization. The nanodiamond crystallinity was confirmed from high-resolution x-ray diffractograms, where Scherrer's equation was applied to (111) and (200) x-ray peaks resulting in grain- size values ranging from 8.5 to 4.0 nm as the methane concentration increased, also corroborated by FEG-SEM images. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3259885]