Abstract: Relativistic electron beam-plasma devices have the potential as one of the newest high-power microwave sources. In this paper, the process of microwave radiation at ω_p from beam-plasma interaction is investigated for n_b/n_p≈ 0.01, where n_b is the beam electron density and n_p the plasma density. The main instabilities have been analysed using the linear and nonlinear (Zakharov) dispersion relations. The dispersion relation of parametric decay by which unstable electrostatic waves are converted into electromagnetic waves has been derived. From this, the radiation rate of a beam-plasma system has also been obtained. Finally, we model how the energy flows from beam kinetic energy to plasma waves, then to EM radiation and plasma heating.