In this paper, the cogging torque in coaxial planetary magnetic gear (CPMG) is investigated. The known method for estimation the cogging torque fundamental frequency in permanent magnet synchronous machines is adapted to the double-rotor structure of the CPMG. The work shows how various design parameters affects the fundamental frequency of cogging torque. Particular attention is given to the relation of cogging torque amplitudes between the low- and high-speed rotors of the CPMG. Results obtained in this paper validated by time-dependent numerical calculations coupled with the law of motion as well as laboratory tests of the prototype.