Abstract: The permeability of fibrous reinforcement is the important parameter for studying and simulating RTM process. In this paper, the unidirectional glass fabric was used to determine the permeability of different kinds of layers under constant injection pressure. During the experiment, the long axis and the short axis of the ellipse, which was formed by the resin flowing in the fabric, were recorded, and the Darcy's law was used to calculate the permeability. The effect of layers on permeability was studied. The results indicate that the orientation of layers influences the value and principal direction of permeability, and it can be determined by the superposition of the ellipse of each single layer.