In this paper, numerical modeling and testing of a complex shaped remotely-operated vehicle (ROV) are shown. The paper emphasizes on systematic modeling of hydrodynamic added mass using computational fluid dynamic software WAMIT^TM on the open frame ROV that is not commonly applied in practice. From initial design and prototype testing, a small-scale test using a free-decaying experiment is used to verify the theoretical models obtained from WAMIT^TM. Simulation results have shown to coincide with the experimental tests. The proposed method is able to determine the hydrodynamic added mass coefficients of the open frame ROV.