In this paper, the influence of magnetic material degradation on the optimal design parameters of electromagnetic devices is presented. The investigation is carried out by incorporating the mechanical stress in the constitutive laws of the magnetic material, i.e., single-valued B-H and iron loss characteristics, by means of a parametric magneto-mechanical model. Then, an optimization problem can be formulated for redesigning the electromagnetic device to reach the predefined target performance. As an application, a widely used electromagnetic device, a surface-mounted permanent magnet synchronous machine, is studied. The presented results depict that the electromagnetic device can be easily redesigned by adapting the values of some flexible design parameters to compensate the effect of magnetic material degradation and to achieve the target performance. This paper can help the designers of electrical machines to improve the quality of the machine design.