Biaxial tensile tests using five kinds of different stress ratios on two kinds of ETFE foils with different thickness were carried out to obtain the stress strain relationship. The equivalent stress of ETFE foils at yielding point was calculated, and the possibility of using Mises yield criterion was discussed. The elastic modulus and Poisson ratio through biaxial tensile tests were obtained, which were compared with the results of uniaxial tensile tests. It is found that the yield condition of ETFE foils can be described by the Mises yield criterion. The elastic modulus and Poisson ratio from biaxial tensile tests are close to the data from uniaxial tensile tests.