This paper proposes high-torque-density dual-stator permanent magnet machines with biased permanent magnet (PM) excitation in the inner stator. The developed machine can be geometrically considered as an outer-rotor stator PM machine plus a separate outer stator similar to the conventional fractional-slot machines. Consequently, the proposed designs feature two different stator structures. Meanwhile, two sets of armature windings are employed to improve the space utilization ratio and torque density. The machine topologies and operating principles are first described. In addition, the analytical models of the machine are introduced, which are utilized to optimize the stator/rotor pole combination as well as the power splitting ratio between two stators. This design optimization is performed in order to maximize the torque capability with the constraint of copper loss. The electromagnetic characteristics of the proposed machine with different inner stator structures are evaluated and compared by the finite-element (FE) method. Finally, an optimized dual-stator biased PM machines prototype is manufactured and tested to verify the FE analyses.