[en] Antiseismic design of Boiling Water Reactor core is one of the most important parts of BWR nuclear power plant design. It is very difficult to investigate the dynamic behavior of the reactor core, because the core consists of many fuel assemblies and is in water. The difficulty mainly depends upon the fact that the reactor core is not only considered as the multidegrees of freedom system, but also the interaction between the water and the fuel assemblies must be taken into consideration. Assuming the flow of the water is two-dimensional, the fluid force on the fuel assemblies is obtained by applying potential theory. The dynamic equations of the fuel assemblies are obtained by using these external hydraulic forces. In addition, results of tests on fuel assembly models are presented and compared with the above calculations. (Auth.)