CHA-type SAPO-34 and SSZ-13 zeolites with different homogeneous crystal sizes were prepared and used as fillers for fabricating polyethersulfone (PESU) based mixed matrix membranes (MMMs). These CHA-type crystals and the obtained MMMs were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). H-2, N-2, CO2 and CH4 gas permeation properties of MMMs with 20% and 30% zeolite weight loadings were investigated at 35 degrees C under the pressure drop of 0.3 MPa. All gas permeabilities were enhanced after filling these zeolites into PESU polymer. The defective MMMs were formed when using the smallest 100 nm SAPO-34 crystals. High permselectivity MMMs were obtained with 200 nm SAPO-34 crystals, and high gas permeability MMMs were obtained when filled with 400 nm SSZ-13 zeolites.