Regenerative brake control which is implemented to a railway vehicle traction inverter sometimes causes oscillation of the filter capacitor voltage. Considering the frequency of the sustained oscillation, an analysis in this paper shows that the frequency is not identical a resonant frequency of the LC of the traction circuit. To design a oscillation suppression controller, it is necessary to clarify the oscillation frequency. Therefore, a simulation to clarify the frequency of the continuous oscillation is carried out in this paper. Based on the result of the real time simulation, authors propose a method to design a controller to suppress the electric oscillation of an electric railway vehicle traction system.