A new approach to measure out-of-plane permeability of single ply unidirectional fiber material under tensile load has been investigated. Single ply permeability is an important material parameter in prepreg fabrication processes where the polymer impregnates the fiber reinforcement. Fibers are processed under tension changing the flow behavior of the resin into the porous media. The experimental characterization cell developed for these processes is mounted to a universal testing machine allowing application and continuous variation of the tensile load applied to the fiber sample. The article outlines the setup including the sensing and data acquisition concept, and implements the data reduction procedure based on Darcy's flow in porous media to obtain the transverse permeability as a function of tension loading. Transverse permeability was determined for single glass and carbon fiber tows for a wide range of tensile loading. The smaller diameter carbon tow permeability showed an order smaller permeability compared to glass fibers. In addition, permeability for both materials changed up to two orders as tensile loading was applied. The results can be used to optimize the impregnation behavior of single ply infusion processes. POLYM. COMPOS., 39:3247–3258, 2018. © 2017 Society of Plastics Engineers