Abstract: High power microwave devices are investigated extensively, because of their potential applications, such as advanced radars, electromagnetic warfare systems. However, low efficiency, enormous volume, huge weight and short lifetime limit their applications. In this paper, a coaxial grating slow wave structure backward wave oscillator (BWO) driven by radial beam is proposed. The focusing system is eliminated in the particle in cell simulation, which can reduce the volume and the power loss in practice. The lifetime of the BWO can also be improved with the thermionic radial beam cathode instead of the explosive emission cathode. After optimization, the BWO driven by 460 kV, 6 kA radial beam can produce 1.2 GW at frequency 3.8 GHz, with the efficiency of 43.5%.