A rotor yoke is rotatable about an axis, is adapted for a plurality of permanent magnets to be mounted therein, and includes a central section, a peripheral section, and a connecting section. The peripheral section is formed with a plurality of outer slots for respectively receiving the permanent magnets. The connecting section is formed with a plurality of inner slots, and a plurality of magnetic reluctance units adjacent to outer slots and each including at least two magnetic reluctance slots that are radially aligned relative to the axis. Each of the magnetic reluctance slots permits portions of magnetic field lines generated by the permanent magnets to pass around the magnetic reluctance slots so as to increase density of the magnetic field lines.