Hat-type steel-sheet-pile enables to improve drivability, structural reliability and economical merit compared with traditional U-type steel-sheet-piles. To apply hat-type steel-sheet-piles to port structures, it is required to quantitatively evaluate durability and corrosion protection performance of hat-type steel-sheet-piles protected by heavy duty coating. Heavy duty coating method with polyethylene or polyurethane has high performance of corrosion protection. However, adhesiveness between steel and polyethylene/polyurethane will be locally degraded in the edge of the coating due to corrosion of steel. In view of the maintenance and long term service life, adhesion degradation at the edge of protection sheet is one of important problem. Delamination of heavy duty coating and corrosion of sheet-piles have been examined by accelerated test and exposure test. Moreover, structural performance of deteriorated heavy duty coated sheet-pile was evaluated, and suitable maintenance method of heavy duty coated sheet-pile was proposed by revised deterioration degree. The principal results are as follows; (1) Deterioration model and life time prediction method of hat-type steel-sheet-piles coated heavy duty was proposed, and their durability was estimated to be as well or better than traditional U-type steel-sheet-piles coated heavy duty. (2) Delamination rate of coating material from the edge was estimated to be 3.2mm/year. (3) The structural performance was predicted by acceleration test results and the standard corrosion rate in marine environment.