Hyper duplex stainless steels have been developed in Korea for the purpose of application to the seawater system of Korean nuclear power plants. This system supplies seawater to cooling water heat exchanger tubes, related pipes and chlorine il1iection system. In normal operation, seawater is supplied to heat exchanger through the exit of circulating water pump headers, and the heat exchanged ea water is extracted to the discharge pipe in circulating water system connected to the circulating water discharge lines. The high flow velocity of some part of seawater system in nuclear power plant accelerate damage of component. Therefore, high strength and high corrosion resistant steels need to be applied for this environment. Hyper duplex stainless steel (27Cr-7.0Ni-2.5Mo-3.2W-0.35N) has been newly developed in Korea and is being improved for applying to nuclear power Plants. In this study, the physical & mechanical properties and corrosion resistance of newly developed material are quantitatively evaluated in comparative to commercial stainless steel in other countries. The propel1ie of weld & HAZ (heat affected zone) arc analyzed and the best compositions are suggested. The optimum condition in welding process are derived for ensuring the volume fraction of ferrite(α) and austenite(γ) in HAZ and controlling weld cracks. For applying these materials to the seawater heat exchanger, CCT and CPT in weldments arc measured. As a result of all experiments, it was found that that the newly developed hyper duplex stainless steel WREMBA has higher corrosion resistance and mechanical propcl1ies than those of super austenitic stainless steels including welded area. It is expected to be a promising material for seawater systems of Korean nuclear power plants.