This paper proposes a simple approach on the synthesis of the 2D profiles for the rotors of cycloidal reducers. A 2D rotor (or cycloidal disc) profile (as a complex closed curve) can be defined (as an alternative to the strictly mathematical definition) as the internal bordered envelope of the family of 2D profiles of the stator (or ring gear) obtained with the stator movable (for at least a completely rotation) around the fixed rotor (or planetary gear) by respecting the gearing conditions inside a cycloidal reducer. The more 2D profiles of the stator are used the more accurate definition of 2D rotor profile is obtained. The finding of