In nuclear power plants, the steam control valves are used to control the flow from the steam generator to the steam turbine under various pressure ratios and openings. Serious flow oscillations can occur due to instabilities of transonic flow through the valve. In the present paper, the two-dimensional study of the flow oscillation in the steam control valve is conducted. The main focus of the present study is the scale effect on the characteristics of the flow patterns. Two sizes of the test models are examined both experimentally and numerically. The result that the frequency of the flow oscillation and the conditions which the flow oscillation occurs are changed by the difference of the size. The frequency is inversely proportion to the height of the duct. The operating range with the oscillation is wider in the larger model than in the smaller model.