The present paper proposes a numerical method for calculating the coupled ship motions and sloshing in time-domain. In the present method, strip theory is used to solve the ship motion problem and a finite difference method is used for the sloshing problem; both problems are calculated in time domain. To verify the computational results, first the calculated force due to sloshing was compared to experimental data and good agreement was found. Next, the authors compared the numerical and experimental results for ship motions, and the accuracy of numerical computation was confirmed. Finally, the wave height at certain position in the tank was compared. Even near the natural frequency of the tank, the numerical results based on the present method showed good agreement with the experimental ones. Moreover, a special case when tank is fully filled with liquid was discussed, and both experimental and computational results showed that the influence of liquid cargo is different from the solid one although the tank is completely filled.