In these years off-shore structures are built with various purposes. It is important problem to study about their mooring system. And this study is also important for ships. In this paper it is aimed to study the tension of mooring lines. Mooring tension was formerly calculated by what is called “catenary theory”, but it is calculated here by “corrected theory”, which includes the effect of elasticity of the mooring line. Using this theory, the tension is calculated not only in ordinary state but in severe surroundings of wave, wind and current. And in this paper, there are mentioned some practical applications and results of experiments as follows ; I. Calculating formulas are composed for various situations. The result shows good agreement with experimental one. Calculating formulas are also made in case that a sinker or a buoy is installed in the middle of the mooring line. II. The tension calculated by these formulas is applied to analysis of the motion of a moored body. A simple one of two dimensional model is used, that is semi-submerged circular cylinder. The motion of the moored body and the tension of mooring lines are calculated by numerical integral methods. Experiment is carried out under same conditions. Comparison of these results is fairly satisfactory. III. The dynamical effect and the similarity law of the mooring line are considered and a few conclusions are shown.