In steam generators, the heat exchange process between the primary circuit coolant and the secondary circuit working fluid, and the flow process of the fluid, occur simultaneously. The hydraulic characteristics of the fluid in the steam generator depend on the properties and state of the working fluid, the structure and geometry of the flow path, and the flow form of the working fluid. Hydraulic characteristics not only affect the heat exchange process, but also affect the economy and reliability of the steam generator. The hydraulic calculation of the steam generator is carried out after the structural selection and thermal calculation, but the structural design and thermal calculation also require hydraulic calculation data, so the three often need to be alternated repeatedly to gradually improve the design. This chapter mainly discusses the hydraulic calculation of vertical U-tube natural steam generators.