Polymer membranes such as poly(1-trimethylsilyl-1-propyne)-polyetherimide (PTMSP-PEI), poly(dimethylsiloxane)-polyetherimide(PDMS-PEI) and PTMSP/PDMS-PEI composite membrane were prepared by solution casting method. Thermal property and gas selectivity of the polymer membranes thus obtained were evaluated. The separation properties of the gas mixture(H₂/N₂) through the composite membranes were studied as a function of pressure. The hydrogen selectivity of the composite membranes used in this work increased regularly as the pressure of permeation cell increased. PTMSP/PDMS-PEI composite membrane was shown to possess excellent real separation factor(α=10.2) of H₂/N₂ which is about 2.3 times larger than that of PTMSP-PEI composite membrane. Selectivity of PTMSP/PDMS-PEI composite membrane increased due to the introduction of PDMS chain to PTMSP backbone.