The changes of pore size in woven geotextiles under tensile loads were measured accurately by digital image analysis. The geotextiles were stretched to maintain 3%, 6%, 9% and 12% in-plane uniaxial tensile strains, whose results were compared with those from unstrained specimens. The characteristics of change were figured out by several parameters, including percent open area, pore size distribution and characteristic opening sizes. The percent open area and characteristic opening sizes increase approximately linearly with the tensile strains. Furthermore, the smaller the pores are, the larger the rate of increases will be. The pore size distributions move towards the direction of the larger open sizes when the stain increases. The change of the pore size results from three elements, which are the rotation of slit film, tensile axial strain and lateral strain of the warp film. The analytic solution of the rate of change of pore area under tensile strains was studied. It concludes to a negative correlation between the rate of change of pore area and the length of pore side.