In the investigation, a dumbbell-shape ETFE foil specimen is employed to conduct mono-uniaxial tensile tests, thus obtaining commonly-used mechanical parameters, namely, yield strength, yield strain, tangent modulus and secant modulus. According to the visco-elastic and plastic features of ETFE foils, the equivalent strain energy method is adopted to calculate the equivalent elastic modulus of ETFE foils. It is found that the calculated equivalent elastic modulus falls in between the tangent modulus and the secant elastic modulus. Then, a set-up is developed particularly for the biaxial test of ETFE foils and is used to perform a series of corresponding tests, and the decoupled biaxial elastic modulus is formulated according to the orthotropic elastic film engineering theory. Based on the stress-strain curve of the biaxial tensile test of ETFE foils, the coupled modulus of a 250mm ETFE foil is found to be 1163MPa at the bilateral stress ratio of 1:1. Based on the orthotropic theory, the decoupled modulus of the ETFE foil is found to be 814.2MPa, which is close to the equivalent elastic modulus obtained through the uniaxial tensile test.