In order to investigate the Eigen characters of the anti-rolling tank of an ocean engineering vessel, a model is designed and made using polymethyl methacrylate plates. The model tests are carried out. A program based on the two-phase flow solver InterFOAM is developed using the code OpenFOAM. The parameters of the natural period and damping are predicted. The effects of the model mesh size, damping plates, tank water height on Eigen characters are discussed. The numerical and experimental results show fairy well agreements. The results can be applied to design the anti-rolling tanks.