Nanocomposite membranes were prepared using Pebax 1657, glycerol triacetate (GTA), and alumina nanotubes (ANTs) by solution casting method. Hydrothermally synthesized ANTs were characterized using SEM, FTIR, and XRD, and additional analysis of FESEM and DSC was also performed on the prepared membranes. ANTs loadings and employed solvent type, dimethylformamide (DMF) or ethanol/water mixture, impacts were investigated on CO2 permeability and ideal CO2/CH4 selectivity of the nanocomposite membranes. CO2 permeability of the ANTs - Pebax nanocomposite membranes was improved compared with the neat Pebax membrane. In the 4 wt% ANTs loaded nanocomposite membrane, chain packing and fractional free volume of its polymeric matrix were tuned by GTA addition (10-40 wt%) to improve the penetrants… permeability through. The Pebax membrane loaded by 4 and 40 wt% of ANTs and GTA revealed 73.75 and -4.97% changes in its CO2 permeability and ideal CO2/CH4 selectivity, respectively. Successful ANTs functionalization by 3-aminopropyltriethoxysilane agent (FC-ANTs), for their higher compatibility with the polymeric chains, was confirmed using FTIR analysis. 2 wt% FC-ANTs loaded nanocomposite membranes' ideal selectivity prepared by using DMF and ethanol/water mixture solvents improved by 5.71 and 14.7%, respectively, while their CO2 permeability decreased by 11.95 and 11.8%.