The effects of C(2)H(2)/(C(2)H(2) + N(2)) ratios on the characteristics of carbon coatings on optical fibers prepared by thermal chemical vapor deposition are investigated. The C(2)H(2)/(C(2)H(2) + N(2)) ratios are set to 60, 70, 80, 90, and 100%. Additionally, the deposition temperature, working pressure, and mass flow rate are 1003 K, 133 kPa, and 40 sccm, respectively. The deposition rate, microstructure, and electrical resistivity of carbon coatings are measured. The low-temperature surface morphology of carbon-coated optical fibers is elucidated. Experimental results indicate that the deposition rate increases with increasing the C(2)H(2)/(C(2)H(2) + N(2)) ratio, and the deposition process is located at a surface controlled regime. As the deposition rate increases, the electrical resistivity of carbon coatings increases, while the ordered degree, nano-crystallite size, and sp(2) carbon atoms of the carbon coatings decrease. Additionally, the low-temperature surface morphology of the carbon coatings shows that if the carbon coating thickness is not smaller than 289 nm, decreasing the deposition rate is good for producing hermetic optical fiber coatings. (C) 2010 Elsevier B.V. All rights reserved.