In this paper, description is made on long period oscillation of a floating body when it is moored relatively such soft mooring systems consist of chains, anchors, sinkers and mooring ropes, in accordance with model experiments and computations.
　The floating body is moored to both linear mooring system and weighted system and subjected to regular or irregular waves. It is confirmed according to the model experiments that wave drift force may be calculated according to Maruo's theory and also that slowly varing wave drift force may be calculated by Hsu's assumption.
　Furthermore, wave damping coefficient was estimated accroding to comparison of results both model experiments and computations and the relation between the characteristics of long period oscillation and load-deflection characteristics of mooring systems were examined.