Synthesis and performance evaluation of vertically aligned carbon nanotube membranes functionalized with different amine groups in order to separate H2S from binary mixtures of H2S/CH4 were experimentally studied. For increasing the performance of the membrane, it was functionalized with butylamine, sec-butylamine, dodecylamine and octadecylamine. The effect of using linear amine groups with different carbon chain length and branched amine on increase of H2S permeability and selectivity of membrane was studied. The results showed that using linear amine with shorter carbon chain length is preferred to other amine groups, and the membrane functionalized with butylamine has the maximum selectivity and permeation values for the H2S. These values are reported to be 2.98 and 2.3 times those of obtained for the membrane with no functional group. The results also reveal that diffusivity of a N-2 single gas is 3.22 and 1.72 times of Knudsen's diffusion before and after functionalization, respectively. (C) 2016 Published by Elsevier Ltd.