Unlike the coaxial magnetic gear (CMG), the magnetic geared machine (MGM) has the ac armature winding on the inner part or the outer part. Besides, the PM excitation field of the CMG and the ac armature field of the MGM have different space harmonic components of magnetic field from one other. Hence, the flux modulation characteristics in the CMG and the MGM are different, and the resultant magnetic radial forces at the modulating pieces are also occurred differently from one another. Thus, in this paper, first, the flux modulation characteristics of the PM excitation field in the CMG and the ac armature field in the MGM are presented. Second, the characteristics of the magnetic radial forces at the modulating pieces are analyzed, and the causes of the radial force according to the space harmonic components of the magnetic field are compared in the CMG and the MGM.