This study demonstrates the successful implications of hydrothermal and secondary growth techniques for synthesis of high property Silicalite-1 and ZSM-5 zeolites. To prepare ZSM-5 a membrane, the nanosized Silicalite-1 powder was first synthesized as a seed from a clear solution. The effects of variations in the synthesis time, aluminum content and the number of seeding on the characteristics of ZSM-5 zeolite were studied. It was found that three subsequent seeding steps enhance significantly quality of membranes surface morphology which led to regular growth of ZSM-5 crystals. The zeolite membranes produced were characterized using SEM, TEM, TGA, DSC and N-2 adsorption techniques. At room temperature, permeance of CO2 was higher than those of N-2 and CH4 and the ideal selectivities for CO2/N-2 and CO2/CH4 were 9.6 and 6.5 at mean pressure of 0.5 bar, respectively. Improved zeolite membrane was fabricated by dip-coating of membrane in PDMS solution. Ideal selectivity of produced composite membrane for CO2/N-2 and CO2/CH4 was obtained as 21 and 14.2, respectively. Ideal selectivity of CO2/N-2 and CO2/CH4 and CO2 permeance decreased with mean pressure in the ZSM-5 zeolite membranes whereas, for composite membrane the selectivities and CO2 permeance increased by mean pressure due to adsorption effect in the range of 0.5-1.5 bar and then remained constant by increasing of mean pressure. (C) 2011 Elsevier B.V. All rights reserved.