Steam conditioning valves are used as important control elements in the process industry. They are used to regulate the flow, pressure and temperature of the steam passing through the valve. Valve geometry and trim are important because that is what regulates the pressure and after the pressure regulation, the temperature is also regulated by mixing it with cooling water. It is necessary to choose the type of control valve that will be used, after that an important step for constructing a steam conditioning valve is to calculate the flow coefficient and the required mass flow of cooling water. The flow coefficient is used to define the geometry of the trim/cage for the desired pressure drop and the required mass flow of cooling water is determined with the desired temperature drop in the valve. After that an important part of this work is to check the strength of the valve wall under high pressure and temperature. Special attention is also paid to the gaskets that will be installed, calculation for these gaskets has been carried out. Finally, suitable standardized flanges were selected for the construction.