Abstract: The virtual cathode oscillation in relativistic electron beams is studied by a 1-D electrostatic particle simulation code with the finite-size-particle model. The calculation results show following conclusions; When injection current is less than the space charge limiting current, cicctron beam propagates stably and transsmits completely. When injection current exceeds the space charge limit, ils propagation is unstable, a part of electrons reflect and the other electrons transsmit. The position and potential of the virtual cathode caused by space charge effects oscillate periodically. When the beam current increases, the virtual cathode position is closer to the injection plane and its oscillating region gets narrower, the virtual cathode potential decreases and its amplitude increases, the oscillation frequency increases above the beam plasma frequency.