Mixed matrix membranes (MMMs) composed of polyethersulfone (PES) and MCM-41 mesoporous silicas were fabricated by solution casting method. To investigate the effect of silane coupling agents on gas separation properties, 3-aminopropyltrimethoxysilane (APTMS) and trimethylchlorosilane (TMCS) were separately used for modifying the surface of particles. The prepared membranes were characterized by FT-IR, SEM-EDX, XRD, TGA and stress-strain tests. Moreover, ideal permeabilities of CH4, N-2, O-2 and CO2 through the MMMs were precisely measured. Regardless of the type of particles, the incorporation of MCM-41 into polymer matrix increased the gas permeability of the MMMs compared to neat PES. Adding unmodified particles into a rigid PES matrix led to non-ideal effects such as formation of interfacial nonselective voids. These voids caused a decrease in thermal stability, Young's modulus and gas selectivity, especially at high loadings. On the contrary, the APTMS-modified MCM-41 with polar N-H groups and long side chains had a good compatibility with PES matrix which facilitated the fabrication of defect free membranes with higher thermal and mechanical strength and unique gas separation performance by 250% and 40% increment in CO2 permeability and CO2/CH4 selectivity respectively. However, the TMCS nonpolar groups did not interact with the PES as strong as APTMS while the TMCS silylation could hamper the particle-particle attraction by replacing hydroxyl groups. (C) 2016 Elsevier B.V. All rights reserved.