When ethylene/ammonia (C2H4/NH3) mixtures are used to deposit carbon films by thermal chemical vapor deposition (CVD), effects of C2H4/(C2H4+NH3) ratios on the deposition rate and microstructures of carbon films are investigated. Experimental results reveal that the deposition rate of carbon films increases with the C2H4/(C2H4+NH3) ratio, and also, raises with the residence time, deposition temperature, and working pressure. The kinetics of this thermal CVD process is discussed. The deposition rate of carbon films is proportional to the C2H4/(C2H4+NH3) ratio with a power of first order, which is resulted from the adsorption of remaining precursor gas C2H4 on the silica glass plate substrate. Few nitrogen and hydrogen atoms are incorporated into carbon films. The activation energy (=152 kJ/mole) of carbon deposition is related to the activation energy of C2H4 dissociation. The crystallinity, degree of ordering, and nano-crystallite size of carbon films decrease with increasing the C2H4/(C2H4+NH3) ratio, while the fraction of sp3 carbon sites of carbon films increase. Finally, the results of thermal CVD carbon deposition using C2H4/NH3 mixtures are compared with those using CH4/NH3, C2H2/NH3, and C2H4/N2 mixtures.