Influence of biaxial stresses applied perpendicularly to the [100] loading axis on the theoretical tensile strength is studied from first principles. Ten crystals of cubic metals and three crystals of diamond ceramics were selected as particular case studies. Obtained results show that, within a limited range of biaxial stresses, the tensile strength monotonously increases with increasing biaxial tensile stress for most of the studied metals. Within the range, the dependence can be approximated by a linear function. Beyond the range, the dependence shows a maximum that usually appears in the tensile range of biaxial stresses. On the other hand, some of the materials (Si, Ni, Cu, and Ge) exhibit a maximum tensile strength at nearly uniaxial stress state, and the superposition of both tensile and compressive biaxial stresses reduces the tensile strength. Unlike the other crystals, diamond revealed a maximum under compressive biaxial stress.