Abstract: The performance of the Ka-band gyro-TWT has been evaluated using the self-consistent nonlinear particle-tracing. The normal mode expansion is used to solve the Maxwell equation with source. Through expand the electromagnetic field in guider-center coordinate, and combing with the electron motion equation, the self-consistent nonlinear equation is induced. The velocity spread, dielectric loading, distributed dielectric loading are taken into consideration to modify the self-consistent equation. The relation of output power with current, velocity spread, dielectric loading, and distributed dielectric loading is given through the numerical calculation. Combining the small signal analysis of self-oscillation, the optimum parameter under stable working point can be got from the nonlinear analysis.