Additive Manufacturing (AM) which is also known as metal 3D printing technique is one of the promising manufacturing processes due to the capability to process a complex geometry component. This is implemented in wide range of applications in various industries such as automotive, aerospace, power plants, etc. The aging nuclear power plant components and the obsolescence of those components has become a concern in this industry, and AM has come as an alternative solution for this matter. The Board on Pressure and Technology Codes and Standards (BPTCS) and Board on Nuclear Codes and Standards (BNCS) Special Committees started to study the application of Powder Bed Fusion (PBF) technique for pressure retaining equipment made from UNS S31603. Also, later Korean International Working Group (KIWG) was also started a Task Group on Additive Manufacturing for Valves which focusing on Powder Bed Fusion (PBF) and Direct Energy Disposition (DED) process for pressure-retaining valve manufacturing especially for nuclear power plant application with the same material. However, the poor mechanical properties and performance, especially fatigue strength of AM materials become a concern due to the defects and flaws as the results of layering and multiple interfaces and welding related discontinuities. In this study, the fatigue strength of PBF and DED manufactured and Ultrasonic Nanocrystal Surface Modification (UNSM) treated UNS S31603 austenitic stainless steel was investigated.