In LMR (Liquid Metal Reactor) core seismic behavior, the fluid coupling between closely spaced hexagons is known to have a very strong effect on the natural frequencies and the seismic responses. In this paper, an experimental study is carried out to investigate the fluid coupling effects with reduced scale mock-ups with three types of hexagonal duct systems and three types of input seismic motions such as the ElCentro 1940 time history, the artificial time history (Reg. 1.60), and the Kobe time history by using a one-dimensional shaking table facility. The results of the experimental studies are compared and discussed with those of the analyses performed by using the SAC-CORE code, which implements a numerical algorithm of the CFAM (Consistent Fluid Added Mass) matrix proposed in a previous paper. From the results, it is found that the fluid coupling between closely spaced hexagons becomes stronger with an increasing number of the neighboring hexagonal ducts, and this causes the natural frequency to be significantly changed and it affects the seismic time history responses too.