Abstract: Coupling of higher order transverse magnetic modes TM310 in microwave coaxial cavity of klystron with TE10 mode of rectangular waveguide is analyzed based on power-voltage and LC equivalent-circuit methods respectively. Gap impedances of output cavity were calculated by using of electromagnetic fields which was point-by-point took out from PAC3D of software ISFEL3D. Results obtained in the two methods made an agreement. It is found that larger gap impedance can be obtained by employing calculated cavity configuration.