In this work, mixed matrix membranes (MMMs) were prepared by incorporating nano-zeolite NaX into poly (ether-block-amide) (Pebax-1657) as an active layer on the polyethersulfone (PES) membrane as a support layer. The influence of filler content, zeolite particle size and operating conditions on the gas separation performance of the prepared membranes were studied. In order to achieve this aim, the zeolite NaX nanoparticles, which were fabricated by the hydrothermal technique using microwave heating, were uniformly distributed in the polymer solution by a two-step procedure for preparation of the membrane casting solution and the mixed matrix membrane was fabricated via the phase inversion technique by solvent evaporation. The prepared membranes were characterized by the ATR-FTIR, AFM, SEM, DSC and contact angle analyses. The gas permeation results revealed that incorporation of nanozeolite NaX into the Pebax-1657 membrane decreased the gas permeability for all tested gases, while the ideal selectivity for CO2/N-2 and O-2/N-2 significantly enhanced. Also, the mixed matrix membranes containing nano-sized zeolite had superior gas separation performance in comparison with the micro-sized zeolite loaded membrane. It was found that the CO2/N-2 selectivity enhanced from 98.0 to 121.5 with increasing the feed pressure, while the O-2/N-2 selectivity decreased from 7.8 to 4.2. For CO2/N-2 separation, the membrane containing 2 wt% nano-zeolite at the operating pressure of 7 bar and temperature of 25 degrees C overcomes the Robeson's upper bound and showed fairly good gas separation performance, compared with the similar Pebax mixed matrix membranes in the literature. (C) 2016 Elsevier B.V. All rights reserved.