The characteristics of multi-surface liquid motion in a rectangular tank are investigated. The rectangular tank is divided into two parts by a baffle, which is vertical to the bottom of the tank and does not reach the bottom. The cases that two liquid surfaces have different areas from each other are mainly interested. Boundary element method is applied to the Laplace equation with linearized boundary conditions. In order to confirm the validity of the calculation, the experiment using the model tank is conducted. A good agreement was found between the theoretical and experimental results. Effects caused by the position of the baffle, and the distance between the baffle and the bottom, upon the natural frequencies and corresponding mode shapes are discussed.