Characteristics of nano-crystalline diamond (NCD) thin films prepared with microwave plasma chemical vapor deposition (CVD) were studied in Ar/H-2/CH4 gas mixture with a CH4 gas ratio of 1-10% and H-2 gas ratio of 0-15%. From the Raman measurements, a pair of peaks at 1140 cm(-1) and 1473 cm(-1) related to the trans-polyacetylene components peculiar to nano-crystal line diamond films was clearly observed when the H-2 gas ratio of 5% was added in Ar/H-2/CH4 mixture. With an increase of H-2 gas content up to 15%, their peaks decreased, while a G-peak at roughly 1556 cm(-1) significantly increased. The degradation of NCD film quality strongly correlates with the decrease of C, optical emission intensity with the increase of hydrogen gas contents. From the surface analysis with atomic force microscopy (AFM), it was found that grain sizes of NCD films were typically of 10-100 nm in case of 5% H-2 gas addition. (c) 2006 Elsevier B.V. All rights reserved.