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Remotely operated vehicle (ROV) dynamics is affected by hydrodynamic forces such as added mass and viscous drag force. Both of added mass and drag force coefficients can be measured by a set of Planar Motion Mechanism (PMM) tests; however, it is costly. In this study, an economic method for identifying hydrodynamic parameters of ROV is given. A simplified nonlinear ROV dynamics model with twelve unknown hydrodynamic parameters in six degrees of freedom was derived for simulations. The identification is based on experimental data obtained by projective mapping method which is utilized to measure the planar motions of a ROV. Then least-squares optimization is performed by comparison between the theoretical simulations and the actual motion measurements. But, an optimization computation involving a large number of parameters is likely to get trapped in a local minimum. Therefore, to reduce the number of parameters to be optimized, some models of simple motions such as surge, sway, surge and sway, yaw, and heave are decoupled from the ROV dynamic model. A commercial ROV ¡§Seamor¡¨ that equipped with two vertical thrusters, two horizontal thrusters, and a depth sensor is used for identification. The experiments for measuring ROV motions of surge, sway, surge and sway, and yaw were performed in a swimming pool in National Sun Yat-sen University. A video camera is utilized to capture ROV for position estimation using projective mapping method. As for the heave motion of ROV, the experiment was conducted offshore the Shio-Liu-Qiu Island and the ROV depth was measured by an onboard pressure sensor. Then, optimal hydrodynamic parameters are identified in sequence of surge, heave, yaw, sway, and then surge and sway. Verification experiments were performed and the simulation results with the optimum values of hydrodynamic parameters show good agreement with the measured data from verification experiments.