This paper is focused on the on-load back-electromotive force (EMF) assessment in a permanent magnet synchronous machine (PMSM) using Maxwell stress tensor along with the frozen permeability method (FPM). Although the back EMF is only measured at open-circuit condition, it is known that the on-load back EMF is heavily affected by electric loading and magnetic saturation. Since the main torque component of the PMSM is the mutual torque and it is based on the interaction between the back EMF and stator current, any distortion in the on-load back EMF will affect the electromagnetic torque and increase the torque ripple. A misleading on-load back-EMF waveform with some spikes around the commutation points is found if the on-load back EMF is assessed by means of differentiating the on-load PM flux linkage. This way, this paper proposes a new numerical method to assess the on-load back EMF in order to avoid all the shortcomings of the existing method. First of all, a quick review on the results achieved by other researchers is shown. Next, a review on the FPM is done in order to clarify the points where this method will be changed to overcome the drawbacks highlighted previously. Furthermore, a new method is introduced and explained in small details. Finally, the results for different rotor configurations and different electrical loads are presented in order to validate the proposed method’s accuracy against experiments and other numerical techniques.