The separation of CO2/CH4 is reported in detail by using zeolitic imidazolate framework (ZIF-8) membrane which was prepared on 3-aminopropyltriethoxysilane modified Al2O3 tube through microwave heating synthesis. Attributed to the preferential adsorption affinity of CO2 over CH4 and a narrow pore window of 0.34 nm, the ZIF-8 membrane shows high separation performances for the separation of CO2/CH4 mixtures. For the separation of equimolar CO2/CH4 mixture at 100 degrees C and 2 bar feed (1 bar permeate) pressure, a CO2 permeance of 1.02 x 10(-8) mol/m(2)center dot s center dot Pa and a CO2/CH4 selectivity of 6.8 are obtained, which is promising for CO2 separation.