Abstract: The filtration applications of woven geotextiles are typically subjected to biaxial tensile strains. The pore size characteristics are important parameters in filtration design. Biaxial tensile strains can cause variations of pore size characteristics, which results in the failure of filtration. Depending on the microstructure of woven geotextiles, the variations of slit-films and pore sizes are analyzed. The mathematical model for strained microstructure of geotextiles is established. The analytical solutions of several pore size parameters corresponding to biaxial strains are proposed, including percent open area and analytical equivalent opening size. The equal biaxial tensile tests are carried out to verify the analytical solutions. The pore size parameters of three slit-film woven geotextiles subjected to different levels of strains are measured and analyzed through digital image analysis, including percent open area, pore size distributions and O₉₅. The experimental results are compared with the analytical predictions. The analytical results correlate well with the experimental values and the rates of variations, and the pore size parameters change approximately linearly with the equal biaxial tensile strains.