The paper presents a new rotary actuator design-the double rotor permanent magnet synchronous machine (DRPMSM), which is to be used in hybrid electric vehicles (HEVs) as a four-quadrant drive system. The focus in this paper is placed on the electromagnetic design, construction and the experimental validation of the proposed DRPMSM. In contrast to other, this work evaluates the radial-radial electric machine concept with one permanent magnet per pole on the outer rotor. Because of the newly proposed outer rotor design, special attention has been given to investigation of the most appropriate stator-slot/outer-rotor pole/inner-rotor-slot number combination. The DRPMSM performance with six different slot-pole-slot combinations was evaluated based on postprocessing of results from time-stepping finite element analyses (FEA). It was elaborated that the combination 36-8-36 was the most appropriate. It is distinguished by low harmonic distortion in the back electromotive force, high average torque with low ripple, and low iron loss as well. A machine prototype based on the 36-8-36 design has been manufactured and rigorously tested in generator and motor operation.