A numerical model has been developed to investigate the nonlinear sloshing motion in a circular basin in response to forced horizontal oscillation. The method is based on the complete boundary value problem of velocity potential and employs no additional assumptions other than those inherent in the potential theory. Experimental observations are in good agreement with the numerical results indicating that the fluid response, both in terms of wave height and hydrodynamic force, is significantly affected by the higher harmonics when the exitation is in the neighborhood of the first characteristic frequency. Moreover, the numerical analyses revealed that the observed secondary peak in the hydrodynamic force response near the predominant peak is caused by the internal resonance with the pumping mode.