This study shows the influence of the surge tank to the work of hydroelectric power plants, and the goal of building and sizing surge tank. The aim of this study is to show that the construction of this part of hydropower leads to a reduction of unwanted effects of inertia of the water and to avoid water hammer. This study is divided into several sections. The introduction explains the definition of surge tank and its function, and the reason for its construction. The second part talks about hydroelectric power plants in general, and its parts and then are explained a headrace tunnel and the inclined penstock which are important for the sizing of surge tank. The third part explains in more details surge tank and its types. The fourth part is to introduce the surge tanks HPP Rijeka whose input data were used in the calculation. Sizing surge tank and determining the amplitude of critical oscillations follows in fifth part and at the end, there is a final review of the thesis in general.