When the working frequencies of vacuum electronic devices reach the terahertz frequency (0.1–10 THz), the Ohmic loss has a great impact on the vacuum electronic devices. To study the effect of the Ohmic loss on the working characteristic of the vacuum electronic devices in the terahertz band, this paper presents the boundary condition of surface impedance used in the 2.5-dimensional fully electromagnetic particle simulation code UNIPIC, which is verified by simulating the terahertz wave in the circular copper waveguide; the simulation result indicates that the code can correctly simulate the propagation of terahertz waves in the waveguide with an Ohmic loss. Then, the coaxial surface wave oscillators (SWO) with slow wave structures (SWS) made of different metals are numerically studied by using the above code, and the dependences of output power on the SWOs with different metal SWSs are analyzed. Numerical results show that the metal conductivity has a considerable effect on the output power of the device: When the conductance of the metal decreases, the quality factor of the device becomes smaller, the start-up time becomes longer, also the output power of the device decreases also. For the coaxial SWOs operating at 0.14 THz, the output powers from the copper and stainless steel SWSs are reduced by 13.4% and 63.9%, the start-up times of the devices are delayed by 0.4 ns and 15 ns, respectively. Meanwhile, the working frequencies of the devices with the SWSs made of different metals keep unchanged.