The aim of this research is to find a feasible method for the estimation of the tensile behaviour of woven membrane construction materials under bi-axial loading. During the model development, the woven membrane materials are treated as orthotropic, elastic and linear. Based on the constitutive relationships for orthotropic and elastic materials, two equations sets are deduced for the estimation of modulus in the warp and the fill direction respectively when suffering from the bi-axial loading. To checkout the validity of the estimation, seven groups of biaxial tensile tests with stress ratios of 1:0, 1:1, 2:1, 5:1, 1:2, 1:5 and 0:1 have been carried out on two kinds of PVCcoated woven membrane materials. From the comparison of the experiments and the prediction, it can be noticed that the model can achieve ideal results when it comes to the estimation of modulus in the leading direction, which is the direction with a higher stress than the other one. Therefore, based on the hypothesis of orthotropic, elastic and linear materials, there is an easy way to estimate the bi-axial tensile properties of woven membrane materials in the leading direction only through the uni-axial tensile tests.