In this paper, dynamic tensions and motions of mooring chains are investigated experimentally and theoretically. Amplitudes and phases of the tensions at the end of a mooring chain and motions at its middle point are measured with the forced heaving test technique. Observations of the motions reveal that the chain oscillates in one of four conditions, namely, Quasi-static condition, Harmonic oscillating condition, Snapping condition and Free-falling and snapping condition. The results of experiments are compared with those of calculations, and the following conclusions are obtained. In the low frequency range in which the chain oscillates in the condition of Quasi-static or Harmonic oscillation. 1) The tension amplitudes nondimensionalized by an initial tension depend on apmlitudes and accelerations of the oscillation and depend slightly on D value ( D = wX ₀/ T _{H ⁰}, w =weight per unit length of the chain in water, X ₀=horizontal length of catenary, T _{H 0}=horizontal component of initial tension). 2) Calculated results by an approximate calculation method assuming a prescribed mode and amplitude of the motion show good agreement with those of experiments. In the high frequency range in which the chain oscillates in the condition of Snapping or Free-falling. 3) The maximum values of nondimensionalized tension amplitudes depend on the accelerations of the oscillation and D value. 4) Calculated results by the lumped masses method are in good agreement with those of experiments.