Abstract: A rectangular storage tank with horizontal baffles was taken as an example in this paper to investigate the nonlinear liquid sloshing problem. The nonlinear damping ratio contributed from horizontal baffles is derived by the potential flow and virtual work theory, and the derived value is then modified to consider the shift in sloshing frequency of storage tank by horizontal baffles. The effects of length and location of horizontal baffles on the wave height and sloshing force are investigated by the nonlinear multimodal method. Numerical simulation is conducted by FLUENT software to verify the proposed theoretical model. Analyzed results show that the nonlinearity of liquid sloshing is obvious when the horizontal baffles are close to the bottom of storage tank or the baffle length is smaller. The wave height and sloshing force estimated from the nonlinear multimodal method are in reasonable agreement with the numerical simulation results. The wave height is underestimated significantly whereas the sloshing force can be predicted with good accuracy from the linear model prediction. The damping ratio of water sloshing in storage tank will increase when the baffles are closer to the free surface or the baffle length increases, which could result in the decrease in both the amplitude of wave heights and sloshing force. The natural frequencies of water sloshing in storage tank decrease gradually when the baffles are closer to the free surface or the baffle length increases gradually. The decreases in the natural frequency are 5.7% and 28% compared with the original value of water tank without horizontal baffles respectively.