Abstract: A new structure of high power and high frequency microwave oscillator based on the transittime instability is proposed and studied numerically in this paper. Due to its coaxial structure with large radius, the oscillator allows larger input and output power and can work at higher frequency. Without external magnetic field being used for beam focusing, it keeps the simplicity of a conventional transittime oscillator. Using an electron beam of 225kV and 11kA, a stable and monochromatic output with peak power of 1.4GW is achieved by simulation at 8.335GHz with an rms efficiency of 28%.