Nano-diamond films synthesized by chemical vapor deposition (CVD) have attracted a great deal of interest for their smooth surface. The work focuses on the carbon concentration of nano-diamond films deposition and its effects on the microstructure of films. Nano-diamond films were deposited by hot filament chemical vapor deposition (HFCVD) method and effects of carbon concentration on the microstructure of nano-diamond films were characterized using scanning electron microscope, atomic force microscope, Raman spectroscopy and X-ray diffraction (XRD). The results indicate that the grain size decreases from 90 nm to 30 nm, the roughness decreases from 52.3 nm to 30.6nm, the line width of peaks of Raman and XRD becomes broader and the intensity ratio of diamond decreases with the increase of carbon concentration from 2% to 3%. The sp(3) carbon phase decreases while the sp(2) carbon phase increases in nano-diamond films with the decrease of grain size, when the carbon concentration ratio increases. (C) 2012 Elsevier B.V. All rights reserved.