In the foregoing reports, the theoretical and experimental studies are carried out on the behavior and the maximum tension of mooring chain which is anchored in the slack condition and is forced to oscillate vertically at its upper end. In this paper, the maximum tension of chain forced to oscillate horizontally and the minimum tension which is related to the snap generating condition are investigated. At first the oscillation of the mooring chain is idealized as a single-degree-of-freedom motion of a lumped mass and the dynamical equation of motion is solved over a broad range of amplitudes and periods. The values of the maximum and minimum tensions of chain calculated by this method coincide with those of experiments fairly well. Secondly the mechanisms which generate the snap tension of mooring line are discussed and the snap generating conditions are derived analytically and are verified experimentally. Finally the method for calculating the dynamic tension of slack mooring chain are summarized for the practical use.