Abstract: tension-leg platform (TLP) or extended tension leg platform (ETLP) is a vertically normally used for the offshore production of or , and is particularly suited for water depths greater than 300 meters and less than 1500 meters. The platform is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. A feature of the design of the tethers is that they have relatively high (low ), such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production on deck (connected directly to the subsea wells by rigid risers), instead of on the . This allows a simpler and gives better control over the production from the or , and easier access for down whole intervention operations. In this paper a numerical study for a square TLP using modified Morison equation was carried out in the time domain with water particle kinematics using Airy’s linear wave theory to investigate the effect of changing water density on the mass matrix of TLP's and the dynamic behavior of TLP's. The effect was investigated for different parameters of the hydrodynamic forces such as wave periods, and wave heights. The numerical study takes into consideration the effect of coupling between various degrees of freedom. The stiffness of the TLP was derived from a combination of hydrostatic restoring forces and restoring forces due to cables. Nonlinear equation was solved using Newmark’s beta integration method. Only uni-directional waves in the surge direction was considered in the analysis.