Abstract: A new terahertz extended-interaction oscillator with disk-loaded waveguide is presented in this paper. The device consists of an input cavity resonator with disk-loaded waveguide, a drift tube and an output cavity. A two and one half dimensional electromagnetic particle-in-cell simulation code is used to investigate it and the results demonstrate that the total length of the slow wave structure, the length of the drift tube and the height of the output cavity affect the output power and frequency remarkably. It is found that under the ondition of the electron beam with voltage 27.7 kV and current 20 mA, peak output power of 442.1 mW at 730.86 GHz is generated.