A steam control valve is used to control the steam flow from a steam generator to a turbine in power plants. The flow field in the steam control valve is usually complex and transonic. A flowinduced vibration can occur in the valve and may cause unexpected shutdown. An experimental study is conducted to clarify the characteristics of the flow in the steam control valve. In the present study, scale effects on the condition of occurrence of various flow patterns are investigated. Experimental results have shown various flow patterns including new patterns which have not been reported. Additionally, even under the same operating condition, different flow patterns have been observed under a certain range of condition. This effect was found to be comparable with the scale effect.