This paper describes the application of Trefftz method to the simulation of sloshing phenomenon on the fluid in two-dimensional vessel. The phenomenon is governed by the Laplace equation with respect to the velocity potential. The equation is solved with the adequate boundary conditions in order to estimate potential derivatives on liquid free surface. The free surface profile and the boundary condition on the surface are updated with the derivatives. Since, in Trefftz method, the potential is approximated with the superposition of the regular T-complete functions, their direct differentiation leads to the derivatives. Trefftz method, moreover, can estimate easily not only the first-derivative but also the second-derivative of the potential function. Therefore, we will discuss more accurate estimation scheme of the sloshing phenomenon using both derivatives. Finally, the present method is applied to the simulation of the sloshing phenomenon of the fluid in the rectangular vessel.