Traveling wave tube (TWT) regenerative feedback oscillators have been demonstrated to be very attractive terahertz source devices. The principle and the physical model of this kind of device are analyzed. To see more detailed oscillation process, a 560 GHz folded waveguide TWT oscillator is presented. The results show steady-state oscillation frequencies exist between 550—600 GHz, and a single-frequency oscillation is found near 560 GHz. An oscillation frequency step-tuning behavior with the continuous variation of the beam voltage is discussed.