A new design of an intercept valve assembly of the intermediate-pressure turbine part of greater power output is investigated in terms of pressure losses and flow fluctuations by using measurement on an experimental valve model. In addition, numerical simulations are used to further clarify measured phenomena. For such valve assemblies, it is important to exactly predict pressure losses and avoid danger of vibrations, which are caused by undesirable flow fluctuations, in order to guarantee valve’s efficiency and operational reliability. For this type of valve, it is especially important for turbine operations in partial loads (off-design conditions). Measurements were carried out in the Aerodynamic laboratory of the Institute of Thermomechanics of the Czech Academy of Sciences (IT) in a modular aerodynamic tunnel. Numerical simulations were carried out in the Doosan Skoda Power Company (DSP) by using a package of ANSYS software tools. The experimental valve model is a scaled model of a real valve assembly. It consists of an inlet pipeline, a stop valve and a control valve including its diffuser and outlet pipeline. Measured regimes were defined by a mass flow rate and a control valve cone lift which can be precisely changed. In order to investigate pressure loses, total and static pressures at valve characteristic locations were measured by using Prandtl probes and wall static pressure taps. In order to measure pressure fluctuations, Kulite fast response pressure transducers were used. They were situated near the valve throat where the flow fluctuations, which are strongly related to a flow separation, are the most visible and influential. Measurement results are compared with numerical results and locations with a flow separation were found. In order to reduce this phenomenon, different valve seat angles were also tested. As a result, a valve design could be optimized and, for a pressure loss prediction, a pressure loss model for this new intercept valve assembly could be created. Therefore, pressure losses in similar valve assemblies can be predicted with required accuracy for each new turbine where modern intercept valves are used. This helps to increase steam turbine efficiency and reduce fuel consumption. Based on pressure fluctuations results, operating conditions at which dangerous flow instabilities occur were identified. It was concluded that there is an operating condition border where the flow field starts to be unstable. As a result, the areas of safe and dangerous operating conditions can be predicted so that the operational reliability of the valve can be guaranteed.