Abstract A relatively simple apparatus has been proposed for conducting biaxial-tensile fatigue tests using an annular disk specimen with a reduced testing section. It makes use of a conventional hydraulic testing machine so that the uniaxial force is converted into radial forces extending the disk specimen. Thus, a specified positive ratio of the tangential to the radial stresses can be imposed at the testing section of the disk specimen. In this paper, the optimal configuration of the specimen is investigated. A number of requirements have been identified and expressed in terms of the stresses in the testing section and their standard deviations. Afterwards, these requirements are grouped into a single performance criterion. The finite element method (FEM) is then applied to fulfill the criterion and realize the disk specimen geometry optimization for isotropic elastic materials. The results obtained from the analysis confirm the potential of the optimized specimen for the study of the material behavior under biaxial-tensile fatigue loading, in particular at high temperatures.