Abstract: The high power Ubitron amplifier with coaxial waveguide is researched using the nonlinear theory in three dimensions. The Ubitron amplifier utilizes an annular electron beam interacting with TE01 mode in a coaxial waveguide. By numerical simulations, the output power of 2.24MW with 10.9% efficiency and 35％ bandwidth in Kuband is gained. The results show that the selffields of the electron beam tend to reduce saturation power, narrow the bandwidth and lower gain. So the effects of selffields should be considered in designing the Ubitron amplifier. Using wiggler taper, the efficiency is up to 12.3%.