The pore size of geotextiles and its distribution have been known to be significant parameters related to engineering properties. This paper presents results from a study undertaken to quantitatively evaluate microscopic void structure of a fibrous medium. An advanced image analysis technique was used to observe the evolution of the void microstructure from surfaces of coupons cut from epoxy impregnated specimens of a needle punched nonwoven geotextile. The voids enclosed by adjacent filaments were measured and expressed in terms of the local void ratio and the largest inscribing opening size. The load history of compression and interface shear of geotextiles against a textured geomembrane resulted in distinctive changes of micro-scale pore network.