Abstract: The analytical solution to the post-peak slope of the stress-strain curve of a uniaxially compressive concrete specimen subjected to single shear failure in the form of the shear band exhibiting linear strain-softening behavior beyond the peak stress was generalized as the nonlinear case. Prior to the peak stress, the Scott model was adopted to describe the nonlinear constitutive relation. The nonlinear strain-softening constitutive relation was back-calculated from the derived post-peak slope of the shortest normal concrete specimen. Using the assessed post-peak constitutive relation, the stress-strain curves of other longer normal concrete specimens were predicted. The predicted post-peak stress-strain curves are size-dependent and agree with previous experiments. The assessed average plastic shear strain in the shear band is extraordinarily greater than the maximum axial strain measured in uniaxial compression. If the measured post-peak stress-axial strain curve is seen as a constitutive relation, then the post-peak ductility of concrete columns will be greatly underestimated.