Under severe storm condition, mooring system of a floating body will become taut condition. To estimate motion and mooring tension on such condition, a practical calculation method considering dynamic effect of mooring line is presented. Also experimental study on a floating cylinder moored by relatively small chain is carried out and following conclusions are derived.Based on simple formulations, presented method can predict experimental results with fairly good agreement. On the other hand, results obtained by static calculation disagree with not only mooring tension but also motion of a floating body.