Resin transfer molding (RTM) is a popular liquid composite molding technique, which has been widely used for manufacturing polymer composites. In RTM, permeability of fiber reinforcement is a crucial factor affecting the property of resin flow and consequently influencing the final product quality. Hence, it is essential to estimate the permeability of the fiber during the manufacturing process. In past research, the permeability is estimated online using the flow-front information captured by a CCD camera and the local pressure measured by a pressure transducer array. However, the feasibility of such method relies on the visibility of the melt flow. When even the mold is not transparent or the color of the resin melt is similar to that of the fiber, this method is not applicable. To overcome the limitation of the existing method, a parallel-plate capacitor is developed in this research for measuring the flow front, where the capacitance keeps an approximately linear relationship with the position of flow front. The principle of the measurement is as follows. Before mold filling, the medium between the capacitor plates is mainly air. Then it is gradually replaced by resin melt during filling. Therefore, the dielectric constant of the medium is continually varying, resulting in the variation of the capacitor value, i.e., the capacitance. Based on the resin flow-front position, the flow velocity is calculated and substituted into the Darcy’s law together with the injection pressure. In doing so, online estimate of permeability is available. The proposed method is illustrated with experiments on an RTM process using glass fiber reinforced plastic (GFRP). Keywords: resin transfer molding, permeability estimation, parallel-plate capacitor, Darcy’s law, glass fiber reinforced plastic.