The coaxial Reditron (the reflected electron discrimination tube) is investigated by means of the two-and-one half-dimensional fully electromagnetic model's particle-in-cell simulation.The results of simulation indicate that the oscillation of virtual cathode will be formed in coaxial guide when the current above the space-charge-limiting current is injected.For the given simulation's parameters,the highpower,narrow band,TM₀₁ mode's microwave can be generated.The peak value of instantaneous output power is 8.62 GW with efficiency of 7 to 10 percent.