Abstract: Fuel assemblies of fast breeder reactor (FBR) core submerged in the liquid sodium may vibrate under the earthquake, which possibly leads to structural damage of fuel assembly or structural deformation of the core and affects structural integrity and safety qualification of the core. Fluids make the vibration of the assemblies appear severe nonlinear. So, the seismic-induced fluid-structure interaction is of great importance to seismic capability of FBR core. The calculation method of added mass is a simplified dynamic analysis method in fluid-structure interaction and was firstly proposed by Westergaard. It considers the hydrodynamic pressure of the water to the structure by the equivalent form of mass. Though the equivalent mass principle has been extensively applied in many fields, it is lack of theoretical derivation in detail. In this paper added mass formula is deduced firstly. Then, the formula is applied for the single-fuel-assembly model and two-fuel-assembly model by CASTEM program in air and water. Finally, the frequency and shock force are compared with experimental datum. The results show that the model calculation results agree with experimental ones.