Plasma structure and electron ring formation in a magnetic mirror trap under electron cyclotron resonance (ECR) conditions is studied numerically through a simulation of the Newton–Lorentz equation by a particle-in-cell method. The plasma self-generated electric field is calculated through a numerical solution of the Poisson equation with the help of the fast Fourier transform technique. The zone where the ECR conditions are fulfilled is almost of a one-sheet hyperboloid shape. It is shown that in the middle transversal plane of the trap, a stable gyrating electron ring is formed. Some trajectories of the hot and superhot electrons comprising the ring are visualized.