Abstract: Based on uni-axial tensile testing experiments of PES/PVC membrane materials, tensile performances of the samples were analyzed in seven in-plane directions including 0°, 15°, 30°, 45°, 60°, 75° and 90°. It is disco-vered that the stress-strain behaviors of the samples can correctly represent the elastic properties of the membrane material after three cycles of tensile loading and unloading. The results show that the elastic performance of the materials can be described by the constitutive relations of the orthotropic materials. In order to predict the tensile strength of the membrane materials under the conditions of complicated stress, the rupture mechanisms of the materials and the corresponding strength criterion were discussed. The analytical results indicate that Tsai-Hill criterion is applicable under the pure tensile or pure shear mode. However, it is less suitable under the tensile-shear mixture mode.