The High-Temperature Engineering Test Reactor (HTTR) is a 30-MW(thermal) helium gas-cooled reactor that uses a prismatic block. The core bottom structure (CBS) of the HTTR consists of an arrangement of graphite components, and it supports the core elements within the reactor vessel. Vibration tests are performed with two scale models to clarify the seismic response of the CBS. The vibration characteristics of the CBS are clarified quantitatively, and the structural integrity of the graphite components is confirmed.