Abstract: Ultrananocrystalline diamond (UNCD) films were prepared by microwave plasma chemical vapor deposition using argon-rich CH4/H2/Ar plasmas with different H2 concentrations from 5 to 20%. The influence of the H2 concentration on the microstructure, morphology and phase composition of the UNCD films was investigated by SEM, XRD, surface profilometry and Raman spectroscopy. It is found that the grain size and surface roughness increase with the H2 concentration. The grain size of the UNCD is less than 6 nm when the H2 concentration is less than 10% and it is still less than 10 nm even when the concentration is 20%. The thickness of the UNCD films is 1.75, 1.80, 1.65 and 2.9 μm using H2 concentrations of 5, 10, 15 and 20%, respectively. All the films are dense and compact in the cross section, and smooth on the top surface.