This paper presents a simple and fast method to include the effect of liquid tanks of a vessel in the prediction of the vessel motions. The effects are expressed in terms of modifications to the added mass and stiffness matrices of the vessels calculated with the liquids in the tanks assumed being rigid. The flows in the liquid tanks are solved using a panel method based on the desingularized boundary integral equations (DBIEs). The numerical results were validated by the experiments of a square tank partially filled. An application example for a vessel with two internal liquid tanks is demonstrated.