The quasilinear theory is employed to study the temporal evolution of the cyclotron instability near the ordinary mode in a magnetized plasma in the presence of the rotating electron beam. The positive gradient region of the initial distribution function is eroded by the diffusion process driven by the wave fields. It is found that the wave non-linear effects modify the electron distribution function by transferring resonance particles to the valley between the plasma bulk and the bump on tail. This movement ultimately leads to flattening the distribution function in the interaction region, which is a saturation state.