Abstract: This paper presents theoretical and experimental researches on the effects of axial guiding magnetic field on microwave power radiated from relativistic backward oscillators. 2.5 dimensions particle-in-cell numerical simulations were done at first. When magnetic field falls the competition between TE mode and TM01 mode contributes to decrease the pulse power of microwave. In addition, cyclotron resonant absorbing was observed. Experiments were made then, which demonstrates that the experimental results are consistent with those of numerical simulation in some degree.