In this paper, a particular class of smooth ultrananocrystalline diamond (UNCD) films synthesized by home-made microwave plasma jet chemical vapor deposition system (MPJCVD) with gas chemistry of Ar-MCH(4)-10%H(2) is presented. This synthesis by MPJCVD yields UNCD films identical to those UNCD films fabricated with Ar/CH(4) chemistry by MPCVD, but using relatively low Ar introduction, low pressure, and low power due to the focused microwave plasma jet enhanced the dissociation of react gases to form energetic species during the deposition. The transition from microcrystalline to ultrananocrystalline diamond films grown from Ar/Ar+H(2) 0% to 90% plasmas using MPJCVD has been systematically studied. The results of this study showed that the grain size, surface roughness, and sp(3) bonding carbon concentration in the films decreased with the increase in Ar concentration. The reason is due to the great increase in renucleation during the films growth. The TEM images clearly exhibited the grain size of the films (Ar/Ar+H(2): 90%) in the range of 3-8 nm. The near-edge X-ray absorption fine structure spectrum also exhibited the clear bonding characteristics of diamond. Moreover, the plasma precarbonization was employed by MPJCVD prior to UNCD films synthesis in order to markedly enhance the smoothness of UNCD films. The UNCD film was synthesized via unique MPJCVD-enhanced nucleation and growth, which produced films with high growth rate (315 nm/h), smooth surfaces (similar to 11.7 nm rms), and extremely fine-grained (3-8 nm) distribution in the whole film. (C) 2011 Elsevier B.V. All rights reserved.