Abstract: An L band all cavity axial extraction relativistic magnetron is designed in this paper. The microwave from two adjacent cavities is coupled into an axially oriented coaxial sector waveguide through radial slots on the cavities. And two neighboring slots locate symmetrically about the center axis of the inner wall of the sector waveguide. When the magnetron operates in the ode, the fundamental mode (TE11) will be excited in the sector waveguide. In an N cavity magnetron, N/2 coaxial sector waveguides are employed to extract microwave power. Particle-in-cell simulation is carried out to optimize the parameters of the magnetron. By employing an electron beam of 600 kV and 6.3 kA, the magnetron could generate a microwave output power of 1.89 GW, corresponding to a power conversion efficiency of 50%, and the frequency is 1.57 GHz. This is a preferred structure for a compact relativistic magnetron because only three sector waveguides are needed to be added outside the magnetron cavity.