A number of studies on the dynamic structural analysis of large scale floating structures with length or breadth to the order of several thousands of meters have been done by several authors, as reviewed by MIYAJIMA et al.. Most of the studies deal with the basic characteristics of the vertical vibrations of the structure modeled as a beam or plate on an elastic foundation. Especially SUZUKI and YOSHIDA have given relatively precise prediction for the deflection and stress of the structure at the lower frequency zone by deriving the analytical solution of such models. This paper firstly deals with the dynamic response and strength of a very large floating structure (about 4560m × 1000m) using the simple beam modeling by the analytical method. Effects of boundary conditions and structural parameters on characteristics of dynamic behavior of the structure are discussed including the characteristic wave number derived by SUZUKI and YOSHIDA. Finally, the dynamic behavior for the same structure replaced with a plane framework is also investigated by applying FEM. The modes and natural circular frequencies obtained are compared with those for the structure modeled as a beam on an elastic foundation. It is also pointed out that not only vertical bending vibrations but horizontal bending vibrations and longitudinal vibrations should be considered at the lower frequency zone.