Abstract: In this paper the dynamical behavior of the electron beam in the cross field planar diode is investigated by means of the one dimensional particle in cell simulation. The results of simulation indicate that the injected current have a critical value. The electron beam will transit to the turbulent behavior from the laminar behavior when the injected current exceed the critical current. When we consider the relativistic effect, this transition to turbulence is very evident, and the value of the critical current suddenly decrease, and don’t, this transition to turbulence is very evident, and the value of the critical current suddenly decrease, and it don’t simplely increase with the external voltage and magnetic fields increasing.