The boundary element method is extended to simulate the non-linear sloshing of liquid in a three dimensional tank. In the proposed method, higher-order element (cubic spline element) is employed for discretization of the boundary integral equation in order to improve the accuracy of numerical solutions. And, the perturbation method is introduced for the purpose of saving the computing time. The details of the boundary element technique are described in reference to formations of cubic spline element. Several numerical examples are presented for liquid-sloshing in prismatic and spherical tanks subjected to forced oscillations, and are compared with the experimental results, furthermore, the usefulness of the proposed method is discussed.