It is necessary to acquire the mechanical properties of advanced plain-weave fabrics as design parameters for application to ultra-light structures. In this study, biaxial tensile experiments of cruciform specimens with an open hole were conducted to evaluate the strength of coated plain-weave fabrics composed of specific high-tensile-strength fibers. Uniaxial tensile tests of strip specimens were also carried out to obtain the fundamental uniaxial properties of the fabrics. The results of these tensile tests show that the open-hole tensile strength of the fabrics under biaxial loading is approximately equal to the strength under uniaxial load irrespective of biaxial load ratio, and warp directional strip specimens exhibit higher strength than weft directional specimens in spite of the same density of yarn in both directions. There is a partial contribution of misalignment of the weft yarn in the membrane to the loss of strength in the weft direction. The results of observations by microscope and single-yarn tensile tests reveal that the strength difference in warp and weft directions is caused by the degradation of the weft yarn by heating in the polymer film coating process coupled with yarn misalignment.