Abstract: Numerical simulations of the coaxial vircator are presented in this paper. The anode cavity provides a resonant mechanism for the interaction of beams and cavity modes. The peak output power occurred when the character impedance of transmission line matched the diode impedance, and the bandwidth of output microwave of coaxial vircator is much narrow than planar vircator. The scaling relation between impedance of coaxial diode and geometry dimensions was approximately determined by simulation results. Comparing with the positively pulsed coaxial vircator, a negatively pulsed one was higher efficiency and more narrow output microwave bandwidth．