Abstract: The pore size characteristics are the important parameters in filtration design of woven geotextiles. The filtration applications are typically subjected to unequal biaxial tensile strains, which can cause variations of pore size characteristics and result in the failure of filtration engineering. The digital image analysis is adopted to test the variations of pore size parameters under unequal biaxial tensile strains of 3 slit-film woven geotextiles. The parameters include length-width ratio of pores, percent open area, and characteristics opening size O₉₅. The ratios of warp to weft strains are set to be 2∶1, 3∶1 and 4∶1. The analytical solutions of pore size parameters corresponding to the biaxial strains are compared with the experimental results. The results indicate that the percent open area and O₉₅ increase with the tensile strains. The larger the ratio of warp to weft strain is, the larger the rate of the curves of pore size parameters is, and the larger the change of length-width ratio of pores. The analytical results correlate well with the experimental values and the rates of variations, and the pore size parameters change approximately linearly with biaxial tensile strains. And the calculation of pore size in the analytical solutions depends on the area of pores, neglecting the variation of the shape of pores.