Polymeric membranes have lots of applications in gas separation process but Robeson used a trade off line to explain that there is a limitation to develop this kind of membrane. To overcome this restriction, Mixed Matrix Membranes (MMMs) have been fabricated by incorporating the inorganic particles as filler into a polymeric matrix. Although the unique gas separation performance of MMM has gained universal attractions, some of non-ideal effects such as poor interfacial adhesion between polymer and inorganic filler deteriorate the general separation properties. One of the possible ways to solve this problem is using silane coupling agents.In this study, to enhance thepolymer-particle binding and to achieve the perfect dispersion of particles in the Polyethersulfone (PES) matrix, the particle surface was functionalized by 3-Aminopropyltrimethoxysilane (APTMS) and Trimethylchlorosilane (TMCS) coupling agents, separately, and the gas separation properties were compared with unmodified ones (neat MCM-41). The structure, size and modification quality of MCM-41 mesoporous particles were determined by FT-IR, Low angle X-ray powder diffractometry (XRD), N 2 adsorption-desorption,DLS and TEM. Then the characterization tests such as FT-IR, TGA, wide angle X-ray diffractometry (WAXD), SEM and tensile stress-strain were performed for better understanding the polymer-particles interaction, thermal stability, the role of filler in crystallinity, quality of the particle dispersion in composites and finally the mechanical interfacial properties, respectively. The SEM micrographs illustrate that the surface modification with both coupling agents, makes the dispersion better and approximately eliminates the non-selective voids. This phenomenon leads to higher thermal stability and increase in mechanical strength compared to neat PES. The ideal gas (CH 4 , N 2 , O 2 and CO 2 ) permeability measurements for resulting neat PES and MMMs were carried out. Although for almost all of tested gases (at the same filler content), permeability of the unmodified MCM-41/PES is higher than the modified MCM-41/PES, the selectivity shows reverse trend. The corresponding permeability values at 8 bar and 25°Cfor20% wt. APTMS modified MCM-41/PES for CH 4 , N 2 , O 2 and CO 2 are 0.096, 0.11, 0.62 and 3.45 Barrer while the permeability of 20% wt. unmodified MCM-41/PES are 0.13, 0.15, 0.65 and 3.56 Barrer, which compared to 0.054, 0.056, 0.29 and 1.40 Barrer for neat PES, show remarkable higher permeability. At the same loading, the selectivity of APTMS modified MCM-41/PES are CO 2 /CH 4 =35.64, O 2 /N 2 =5.6 and the selectivity of unmodified MCM-41/PES are CO 2 /CH 4 =26.46, O 2 /N 2 =4.21. The improvement in selectivities is attributed to the elemination of non-selective voids and particles better dispersion. Moreover, the unexpected CO 2 selectivity enhancement for MMM with APTMS modified particle can be result of strong interaction between amine groups and CO 2 via formation of surface carbamate groups.It is worth pointing out that with 20 wt% of unmodified MCM-41 silica mesoporous in PES, CO 2 permeability of the mixed matrix membrane is shown to increase by up to 155% compared to the neat PES. Keywords: Polyethersulfone (PES), MCM-41, Mixed Matrix Membrane (MMM), Silane Coupling agent