This paper deals with surface and interface sloshing in a tank containing two fluids of different desities. Scientific interest in this ploblem includes the need to quantify allowance loads on oil water separators and chemical plants. In this study, we give a Hamiltonian formulation of a system of two incompressible irrotational fluids with a dynamic free surface and interface. This study aims to analyze the linearized time history response and the linear properties (vibration mode and effect of surface tension on the natural frequency). As a result, it was revealed that this system has two different vibration modes (surface and interfacial mode) with same wave number and the surface tension causes natural frequency increases. The validity of the theory is verified by experimental results. This procedure can be directly applied to the nonlinear problem of this system.