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Resin flow plays a crucial role in many composite manufacturing processes. The most important parameters used in modeling and designing mold filling are the permeability of the fibrous preform, which is a kind of flow conductance and a property of the reinforcement, and the viscosity of the resin. The extent of chemical reaction, or degree of cure, is also important and causes change of resin viscosity during mold filling. To determine the permeability of fiber preforms, many researchers have been using liquid flow analysis. In this study, a new scheme for determining permeability using gas flow is proposed. In conventional liquid flow methods, radial propagation of the polymer into a porous medium is measured and used to determine permeability, whereas in the gas flow method, the flow rate for several different preform geometries is measured and used. The effectiveness of the gas flow method was verified by comparing it with conventional methods.