Abstract: Elastic constants obtained from uni-axial tensile tests are frequently used to represent elastic properties of architectural membranes reinforced by woven fabrics. However, it has generally been accepted that the elastic constants from bi-axial tensile tests are more applicable in the design of architectural membrane structures. The purpose of this study is to develop a method to estimate bi-axial elastic constants of architectural membrane material by using uni-axial tensile tests. General method for obtaining bi-axial tensile modulus of membrane materials is first presented to show the influences of loading conditions. Then, based on the constitutive equations for 2-dimensional orthotropic material and by taking the loading ratios in warp and weft directions as loading parameters in bi-axial stress-strain tests, a model is developed to build a relationships between the elastic constants of membrane materials reinforced by woven fabrics under both uni-axial and bi-axial tensile loading. Finally, testing data of typical architectural membrane materials obtained from literature, GF/PTFE, and PVDF1202T are used to verify the model. Good agreements have been illustrated to validate the proposed method.