Coupled eigen-value analysis of elastic beam-fluid system using new analytical formulation is presented. The boundary value problem to calculate the eigen-value of coupled system is formulated based on the consideration of dynamic interaction between elastic beam and perfect fluid. The frequency equation of coupled system is derived from BEM-FEM combined method. And coupled eigen-value is solved by Determinant Search Procedure. In this study, the lower coupled frequencies and vibration modes of very large scaled floating structures are discussed through numerical calculations especially.