Fiber orientation is a key parameter affecting the geometrical, hydraulic and mechanical properties of nonwoven materials. The effect of fiber orientation on the pore size has been experimentally investigated based on air-laid, parallel-laid, and cross-laid structures following through-air bonding. It was evident that there is a discernible difference between the mean flow and maximum pore sizes of these nonwoven materials. The influence on pore size was further elucidated by evaluating experimental and theoretical models based on sieving-percolation pore network theory including a model that incorporates directional parameter to account for the effect of fiber orientation. It was established that good agreement with experimental data can be obtained using such a model. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2668-2673, 2010