In order to develop compact, reliable and high power terahertz radiation sources, multi-mode second-harmonic operation in a terahertz gyrotron was investigated theoretically and experimentallyin . The theoretical calculation results show that there are five second-harmonic modes which operate at single-mode oscillation state in the gyrotron by adjusting the design parameters. Experimental results demonstrate that there are four second-harmonic modes (TE16, TE64, TE46 and TE26 ) in the gyrotron operating with a 1.5A/41kV electron beam. Output powers of 1.5, 2.1, 3.0 and 1.4kW can be generated at the corresponding frequencies of 0.391THz, 0.404THz, 0.416THz and 0.423THz, respectively.