As one of the relativistic electron tubes having compact configuration and high efficient output,the relativistic magnetron with direct axial radiation is very attractive in pulsed power and high power microwave fields for industrial and military applications.In this paper,the experimental investigation of a relativistic magnetron with axial TE11 mode radiation is reported.Under a total length of ～ 0.3 m,volume of ～ 0.014 m3,working at an applied voltage of 508 kV and a magnetic field of ～ 0.31 T,the relativistic magnetron radiates a microwave of 540 MW with the TE11 mode at 2.35 GHz in the axial direction.The power conversion efficiency is 15.0％.After a lot of shots,the detected amplitudes of microwaves are nearly the same.The fluctuations of wave amplitudes are less than 0.3 dB.