Three-dimensional nonlinear liquid motion in a partially filled rectangular tank supported by pendulum is investigated. In this analysis, the nonlinearities of the liquid surface oscillation and pendulum motion are considered. Basic equations of liquid elevation and pendulum angle are derived by employing the variational principle. The nonlinear differential equations are derived by applying Galerkin's method to the basic equations. The characteristic of the nonlinear coupled oscillation is discussed. The coupled natural frequency is obtained by linearizing the derived equations. By the coupled natural frequency analysis, it is showed that, when the aspect ratio of the tank is a little less than integer, the three-dimensional liquid oscillation can occur. It is confirmed that the three-dimensional liquid oscillation occurred in numerical simulation and experimental results. A fairly good agreement is found between both results. It is shown that the three-dimensional nonlinear liquid motion plays an important role of liquid oscillation in a rectangular container supported by pendulum.