In general, the roll inertia moment and its longitudinal distribution etc. are not calculated strictly in wave load analysis but are calculated approximately by simplified formula, even though the detailed model is used in structural analysis. Needless to say, it is very important to correctly estimate the mass and the inertia moment of the ship for the motion performance as well as ship structural safety. In this paper, the authors proposed the practical calculation method which could obtain the mass, COG (center of gravity), the inertia moment of the granular/liquid cargo, and these longitudinal distribution from FEM model. Additionally, the comparison of roll inertia moment between the values from proposed method and the classical simplified formula for the ore carrier was conducted. Moreover, the longitudinal distribution of the roll inertia moment and the height of COG were calculated strictly and the influence on the torsional moment in waves was examined. The proposed method can be performed easily, which needs FEM model of a ship and the cargo shape which are normally prepared when direct load structural analysis is conducted.