Boron-doped diamond (BDD) films were grown with different grain sizes. The films were deposited on silicon substrate after a suitable pre-treatment in a hot filament assisted by chemical vapor deposition (CVD) reactor in Ar/H2/CH4gas mixtures. The addition of argon to the growth gas mixture clearly revealed the transition from nanocrystalline (BDND) to ultrananocrystalline (BDUND) diamond films. Raman spectroscopy results of BDD, BDND and BDUND exhibited a good quality diamond films considering the diamond defined peak. Fourier Transform Infrared Spectroscopy (FTIR) spectra indicated carbonyl groups and B-C in BDND and BDUND films, while the microcrystalline BDD films showed only C-H bonds and boron-carbon (B-C), without the presence of oxygen and unsaturated species. Therefore, the carbonyl presence in the nanocrystalline films is mainly due to oxidation of transpolyacetylene present at the film grain boundaries. The transition became pronounced in the gas mixture with 60% of Ar, and the microcrystalline films were totally transformed in nanocrystalline diamond at 70% of Ar.