Abstract: The new apparatus for determining water vapor permeablity of fabric utilizes waterproof vapor-permeable PTFE membrane to cover the base of moisture permeation cylinder,providing the source of saturated water vapor.The dry nitrogen stream is used as the carrier to get the vapor penetrating the fabric away.The water vapor transmission rate can be determined by measuring the relative humidity of outgoing nitrogen stream.The test results indicate that this test method is able to accurately evaluate the water vapor permeability of fabric within 5 minutes,and the coefficient of variance is less than 1% for water vapor permeability of the fabric tested.Therefore,this test method offers short test time,good repeatability,high sensitivity and less cost.It is very useful for routinely testing the water vapor permeability of fabrics for quality control of the textile plants.