The design and simulation of a harmonic gyrotron based on a cusp electron gun is presented. The gyrotron is operated at the 7th electron cyclotron harmonic with an output frequency of ∼390 GHz and mode pattern of TE71. A large orbit electron beam with energy of 40 keV, beam current of 1.5A and an adjustable pitch alpha from 1 to 3 from a cusp gun is used to drive the beam-wave interaction. Smooth cylindrical waveguide and slotted structures have been studied as interaction cavities. Theoretical analysis and numerical simulation results of these structures will be presented.