Abstract: Through changing the number of glass fiber mat layers to control the porosity, the saturated permeability of Windstand^TM glass fiber in-plane and trans directions(x, y and z)was measured by the unidirectional saturated flow method. The influence of porosity, mold dimension and fibers direction (parallel and vertical 2% extra fibers tows) which affect the permeability were investigated. The results indicate that with the decrease of porosity, the permeability drops sharply. The effect of mold dimension has little influence of y direction permeability when porosity between 34.6%-54.7%. Changing 2% extra fibers tows direction (from parallel to vertical), has an effect on permeability when the porosity between 34.6%-54.7%. When porosity range is 45%-55%, the permeability in-plane direction is about 3-7 times as high as that in transone.