Evaluation of sloshing loads is a key point in the design of ship liquid cargo tanks since when the tank is excited at near-resonance frequencies, violent motions of the liquid can occur. This implies large impact pressures and resulting additional forces on the structure, which could possibly lead to structural damage or ship capsize. In this paper a new CFD simulation approach is used to challenge the sloshing problem. The method can treat simultaneously liquid, gas and solid phases. It is essentially a variation of the CIP method with a recently developed CIP-CSL3 scheme for capturing sharply the gas-liquid interface. Results by the original CIP method are also presented. Computations for pressure time histories and free-surface profiles in a 2-D rectangular tank are compared to experimental results. Several frequencies and several filling heights are investigated.