Abstract: A 3 mm low-voltage second-harmonic complex cavity gyrotron with gradual transition is designed and evaluated with a particle-in-cell code. The high efficient gyrotron interaction can be achieved at the second harmonic. The insulation requirements of the gyrotron and the size and weight of the power supply can be reduced by setting the operation voltage at 25 kV. A diffraction-coupled complex cavity operating in the low ohmic loss TE02/TE03 circular modes is introduced to control the mode competition problem and promote efficiency. The characteristics of RF structure, the suppression of parasite oscillations, and the optimization of work parameters are discussed. The PIC simulation results show that the gyrotron can generate 34 kW output power at 94 GHz with 34% operation efficiency.