Boron-doped nanocrystalline diamond (BDND) films were grown on silicon substrates by hot filament chemical vapor deposition in Ar/H2/CH4 gas mixtures. The boron source was obtained from an additional H2 line passing through a bubbler containing B2O3 dissolved in methanol with different B/C ratios. The transition from ultrananocrystalline to nanocrystalline diamond films is clearly shown by the addition of boron dopant to the growth gas mixture. The morphology and structure of these films have markedly different properties. The top view and the cross section of the films were characterized by scanning electron microscopy showing the transition from ultrananocrystalline growth (renucleation process) to a columnar structure of NCD films. Finally, the grain size was obtained from X-ray diffraction patterns of the films. The diamond average grain size increased from 10 to 35 nm for films with 2000 and 30,000 ppm B/C, respectively.