A harmonic multiplying gyro-travelingwave tube (gyro-TWT) has bem designed, fabricated. It triplicates X-band input frequencies to amplified Ka-band output frequencies. Using a self-consistent large-signal theory , a saturated gain of 23.8 dB, a bandwidth of 4.1 %, and an electronic efficiency of 10 % are predicted when an axis-encircling electron beam of 4 % axial velocity spread is assumed. 30 kV, 1 A cusp electron beam is employed. The interaction circuit is atwestage linearly tapered rectangular waveguide with a server where the RF is completely cut-off In the input stage, the fundamental cyclotron harmonic interaction occurs at X-band frequencies and the third harmonic interaction is occurred in the output stage with dtstributed interaction along the circuit. Two-stage linearly tapered gyro-TWT was first demonstrated at NRL , which shows a 20 % bandwidth at Kaband with saturated gain of 25 dB using the fundamental cyclotron harmonic interaction in both stages.