A comparative study of designed values and practical results on fatigue damage of 'POSEIDON' a proto-type floating structure was described in part 5. It was shown that the estimation of response transfer function is important to prediction of fatigue life in part 5. This paper describes effects of hydrodynamic interaction between multiple columns on structural response in waves. Three-dimensional source distribution method was used to estimation of hydrodynamic interaction. Model test on wave exciting force was carried out. Theoretical results and test results were compared and it was confirmed that the technique of entimation is valid. Structural response functions were solved in frequency domain by Finite Element Method. Results of hydrodynamic culculation was used in F. E. M. analysis. Effect of hydrodinamic interaction against structural responses was confirmed by comparison of predicted results and practical data. The fatigue damage was predicted by using to power spectrum density of strain response. This study showed that effect of interaction and wave directional function have influence on prediction of fatigue life.