The yield behavior of an amorphous glassy polymer has been studied with true tensile stress-strain curves, obtained at various crosshead speeds by means of a new experimental method. A constitutive equation from nonlinear viscoelasticity has been used, with the further assumption that the material, during deformation, subsequently follows the following two distinct paths: a nonlinear viscoelastic, and a plastic one. The maximum strain, where this distinction is manifested, has been treated as a control parameter, while the strain rate was experimentally evaluated. The decomposition of deformation has been made with a suitable kinematic formulation, proposed in the literature. The theoretical results describe the experimental curves in detail.