An electric motor rotor includes a core member having a plurality of locating grooves spaced around the periphery thereof and outer rotation-stopping portions, and a magnetic unit arranged around the periphery of the core member and defining a plurality of magnetic zones corresponding to the space between each two adjacent locating grooves. The magnetic unit can be a magnetic barrel having inner rotation-stopping portions for abutting against the outer rotation-stopping portions of the core member. Alternatively, the magnetic unit can be made comprising a plurality of magnets to form the multiple magnetic zones and a hoop for attaching to the magnets to secure the magnets to the periphery of the core member. Using the core member with the said two magnetic units can make two different rotors that have the common advantages of quick assembly, accurate alignment of magnetic zones and cost-effectiveness.