In this research paper, free decay method is proposed to identify the hydrodynamics coefficient of ROVs. This method is preferable for smaller-scale testing as it is a smaller and simpler method. In order to carry the experiment, an experimental rig is designed and constructed. The experimental rig is used to create an underwater environment which is important in the identification of the hydrodynamics coefficient. First, the equations of motion are formulated that can be used to describe the motion of pendulum given a set of parameters which included measured parameters such as mass and length of pendulum and also the unknown hydrodynamic coefficient which are added mass and the damping coefficients. Then, experiments are conducted where the displacement of the pendulum is recorded over time by using camera. The desired parameters are found by finding the combination of parameters such that the mathematical model fits the experimental data. Through this research project, it was found that the results obtained from this method are not reliable as claimed by a few authors. Although the data processing procedure was a success, the results obtained vary widely with different initial condition. This research project shows that in the current state, this method is not a good method in evaluating the hydrodynamic coefficient of the ROVs. Thus, an alternative method should be proposed or improvements should be made to produce a better result.