Modeling the dynamic of a submerged body has received a wide attention in the recent years. The trend is toward modeling the buoyancy and motion of the sub- merged body considering di®erent variables affects on it. The scope of this thesis is modeling the dynamic of a submerged body by estimating its natural characteristic, m,c and k since it is necessary to design offshore structures and highly important in the ocean wave arena. An experimental study is carried out to model the dynamic of a submerged body in its heave mode. Since extracting all parameters from this experimental is not an easy task when measuring low frequencies, a numerical model is developed. The resonance frequency, added mass and the stiffness of buoy is successfully estimated. The nonlinearity of the system is detected here by comparing the estimated dynamic characteristic parameters of a different amplitudes.