Abstract: The equivalent opening size tested under unstrained condition is used in the current retention criterion for design of geotextile filters. However, the equivalent opening size can be influenced by in-plane uniaxial tensile strain. The retention criterion cannot be satisfied when it gets larger, and the permeability criterion and the clogging criterion cannot be satisfied when it gets smaller.The opening size of woven geotextiles can be featured by the pore size distribution and characteristic opening sizes. The opening sizes of two uniaxially stretched woven geotextiles are measured accurately by means of the digital image analysis. The in-plane uniaxial tensile strains of the woven geotextiles are increased to 3%, 6%, 9% and 12%. The percent open areas of the woven geotextiles and three characteristic opening sizes (i.e., O30, O50 and O95) increase with the increase of the tensile strains. Accordingly, the pore size distributions shift to the direction of the larger open sizes. The change rates of characteristic opening sizes approximately linearly increase with the increase of the tensile strains, and smaller characteristic opening sizes have larger increase with the increase of the tensile strains.