The structure of fine-grained diamond films with crystallite size as small as 10-50 nm has been studied with cross-section transmission electron microscopy and Raman spectroscopy. The nanocrystalline films of 1 to 2 μm thickness were grown on Si substrates by d.c. plasma chemical vapor deposition in Ar/CH 4 /H 2 gas mixtures, with methane concentration CH 4 /(H 2 +CH 4 ) varied from 3 to 100%. The substrates were seeded with 5 nm diamond powder to enhance the nucleation density. Submicron thick nanocrystalline films were also grown as a first layer for successive growth of large-grain films. The films demonstrate a kind of columnar growth even for the case of grains with a size of a few tens of nanometers. The crystallites showed many imperfections, twins on (111) planes being the dominant defect type. The twin density is very high, but often they are only a few atomic layers wide. A buffer layer of β-SiC with a thickness up to 400 nm can be observed at the diamond/silicon interface. In addition to the diamond phase, Raman spectroscopy revealed disordered sp 3 and sp 2 carbon phases, presumably located at grain boundaries. High intrinsic stresses in the film prevent the observation of the size-induced low-frequency shift of the fundamental diamond peak at 1332 cm - 1 . The appearance of fingerprints of C-N vibrations and distorted diamond in Raman spectra is discussed.