With electric vehicles (EVs) and hybrid electric vehicles (HEVs) set to grow in the coming years, design optimizations of electric motors for automotive applications are receiving more attention. Under demanding duty cycles, the moving part within a motor, the rotor, may experience high and varying stresses, which may lead to fatigue failure. Therefore, engineers are facing challenges in designing efficient and durable motors, especially for interior permanent-magnet (IPM) motors, in which the rotors have embedded magnets with small "bridges" of laminated electrical steel to keep the magnets in place.Cost-effective stators and rotors are made from electrical steels, with high magnetic permeability and low power losses. However, national and international standards for electrical steels do not specify mechanical properties. Steel producers would normally state typical mechanical properties only, and no fatigue properties are available in published literature.This paper describes typical fatigue performance of an electrical steel grade SURA® M270-35A for EV/HEV motor applications. Focus will then turn to finite element (FE) structural and magnetic evaluations of various IPM rotor designs. Particular attention will be paid to understanding the influence of the magnet pole arrangements, the magnet slot geometry and "bridge" dimensions on rotor fatigue lives and rotor-stator air-gap flux densities.By generating and applying fatigue data for electrical steels, in conjunction with FE-based magnetic, structural, and fatigue modeling, the paper summarizes how various magnet-slot schemes and rotor profiles can be assessed under a typical HEV motor duty cycle. Such understanding provides good pointers to optimized motor rotor profiles with balanced durability and magnetic flux density in the air-gap. The paper demonstrates that, using the electrical steel property data and the proposed design procedure, it is possible to design durable IPM motor rotors that can last much longer than the expected lifetime of an EV/HEV vehicle.