Applying the hydrophobic membrane into the absorber and generator of the absorption chiller can significantly downsize the absorption system. In this study, the permeability of the hydrophobic membrane is evaluated using CO2 prior to the system test. The PTFE flat-type membrane with three nominal pope sizes of 0.1, 0.45 and 1 micron were tested for both flow directions. Results show that the permeability decreases with imposed pressure when the pressure is relatively small, and then tends to be constant. The mass transport mechanism differs for the membrane samples, and higher permeability was obtained with larger pore sizes. The variation of the permeability can be qualitatively explained with the pore size distribution.