In the present paper, the beam-wave interaction behavior in the gyro-TWT amplifier has been studied using time dependent nonlinear single-mode analysis. The time dependent equations of motion which describes the interaction of the electrons with the RF wave are solved self-consistently for the performance evaluation of a typically selected fundamental mode 35GHz gyro-TWT amplifier operating at TE01 mode. The results from nonlinear analysis shows that the present gyro-TWT produces a stable output power of 165kW for 1% velocity spread at 35GHz, with ∼42dB gain, 22% efficiency for a 72kV, 10A gyrating electron beam. The analytically obtained values of RF output power and gain are found to be in agreement with the available published results within ∼10%.