A thin and well-intergrown zeolite T membrane with high CO2 permeability and selectivity was rapidly prepared on seeded tubular supports by the two-step varying-temperature hydrothermal synthesis process (two-step method). The influence of synthesis parameters such as synthesis temperature and crystallization time during the two-step durations on crystals growth and separation performance was investigated systematically. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and gas permeation measurements were used to characterize the as-synthesized membranes. Compared with the conventional hydrothermal synthesis at a constant temperature (one-step method), the two-step method is more effective to synthesize a thin and continuous zeolite membrane layer in short time, which fully covered the support surface. Membrane synthesis temperature during the two-step durations could seriously influence the crystal growth and CO2 separation properties. It was found that a lower temperature favored nucleation, while a higher crystallization temperature promoted crystallization during the two-step method, thus improving zeolite T crystal growth and membrane separation properties. The membranes prepared by the two-step method under the optimized conditions exhibited good reproducibility, high CO2 permeance, and relatively high permselectivity for the separation of CO2 from CH4 or N-2 gas mixtures. For example, CO2 permeance and CO2/CH4 selectivity of the membranes reached 6.2 X 10(-8) mol m(-2) s(-1) Pa-1 and 80 for the equimolar CO2/CH4 binary gaseous mixtures at 35 degrees C, respectively.