Mixed matrix membranes (MMMs) consisting of acid-treated functionalized multi-walled carbon nanotubes (MWCNTs) incorporated into polyimide (PI) polymer were fabricated for gas separation. The functional groups and dispersion properties of the MWCNTs were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the functionalized MWCNTs were cut into short ropes and acid oxidation introduced oxygen containing functional groups such as hydroxyl (OH) and carboxylic acid (COOH) on the surface of MWCNTs. Based on the assumption of strong interfacial interactions between the functionalized MWCNTs and PI matrix, the MWCNTs were first incorporated into poly(amic acid) (PAA) to form a highly dispersed solution, then the PI/MWCNTs nanocomposites were formed after in-situ chemical imidization. Scanning electron microscopy (SEM) indicated that the functionalized MWCNTs were dispersed homogenously in the PI matrix. The gas separation performance showed that the CO2 permeability coefficient increased 292% at 3 wt.% MWCNTs incorporated into the MMMs, and there was a 145% increase in the selectivity of CO2/N-2 and a 144% increase in the selectivity of CO2/CH4. This result demonstrates that the carboxylic and hydroxyl groups have a strong interaction with CO2, which increases the solubility coefficient of the polar gases. The in-situ polymerization approach of fabricated MMMs improves the interfacial interaction between the dispersed nanomaterials and the polymer matrix, which can be utilized in the practical gas separation technology. (C) 2017 Elsevier B.V. All rights reserved.