Humidification and dehumidification are common processes in air conditioning industry. In this study, a new technique-the hydrophobic membrane based air humidification and dehumidification are investigated. The effects of membrane characteristics on moisture exchange effectiveness of the system are studied. A two-dimensional transient model which takes into account the combined mechanisms of Knudsen flow and ordinary diffusion in membrane pores is proposed and validated. Four membranes, 2 hydrophobic treated Nylon and 2 PVDF, are used in the test to validate the model. Then the effects of variations of membrane properties on the system performance and membrane to total resistance ratio, are evaluated. A dimensionless Number of Transfer Units (NTU) can be summarized to govern the moisture exchange performance. Following, a correlation has been obtained to reflect the relations between the moisture exchange effectiveness and the Number of Transfer Units.