Abstract This paper presents the results of the experimental and the theoretical investigation of the dynamic behavior and the tension of a mooring chain oscillating at various frequencies of the wide range in water. The movements of a mooring chain, when forced to oscillate vertically or horizontally at its upper end, are observed by means of cinematography and the dynamic tensions induced are measured simultaneously. The results of those experiments reveal that the chain oscillates in one of the following four conditions as the frequency increases, namely, [I] Quasistatic condition, [II] Harmonic oscillate condition, [III] Snap condition and [IV] Free-fall condition. The maximum amplitude of dynamic tension occurs in condition III] or [IV] and cannot be neglected in comparison with the magnitude of the initial tension of chain. The results of the experiments are compared with the theoretical ones and the following conclusions are obtained.The tension amplitudes depend on the amplitudes and the accelerations of oscillation, D value (D = wXO/THO, w = weight per unit length of chain in water, XO = horizontal length of catenary, THO = horizontal component of initial tension) and the drag and added mass coefficients of chain in water.In the conditions [I] and [II], the tension computed with a theoretical method assuming a mode shape and an amplitude of motion shows good agreements. This approximate calculation method is of practical use to estimate the dynamic tension of chain.Results calculated by the lumped mass method also coincide with those experiments fairly well. Introduction The reliability of a ocean structure moored in the severe ocean environment depends upon the reliability of its mooring system. Therefore, it is very important that the performance of mooring lines subject to the severe environment in the ocean can be accurately predicted. Recent investigations 1 ~ 5 on the mooring line performance have shown that the magnitude of dynamic tension of an oscillating mooring line cannot be neglected in comparison with the magnitude of its initial tension. The dynamics of oscillating chain in the various frequencies of the wide range have not been clarified yet. In this report, these problems are investigated experimentally and theoretically in the case that a mooring chain is forced to oscillate both vertically and horizontally at its upper end in relatively slack conditions in water. The movement of mooring chain is observed by cinematography and the induced dynamic tensions are measured by load cells simultaneously. The results of those experiments reveal that the chain oscillates in one of the following four conditions as the frequency increases, namely, [I] Quasi-static condition, [II] Harmonic oscillate condition, [III] Snap condition and [IV] Free-fall condition. At the conditions [II], [III] and [IV], the dynamic tensions of chain cannot be neglected in comparison with the initial tensions and they become maximum in the condition [III] or [IV].