Automated Ball Indentation (ABI) is one of the most promising miniature specimen techniques to characterize the mechanical behaviour of the materials. Utilization of ABI technique to monitor the mechanical properties of the structural materials used in commercial nuclear power plants is highly appreciable, due to its non destructive and In-situ measurement capabilities. In this book, ABI was applied to investigate the structural integrity of various structural component materials used in both light water and heavy water cooled nuclear reactors. The variation of mechanical properties in localized zones of Dissimilar weld joint (joint of SA 508-Cl 2 reactor pressure vessel nozzle and SS 304 LN pipe line with Inconel weld)and Similar weld joint (joint of SS 304 LN pipe lines)was examined using ABI technique. ABI technique was also utilized to investigate the mechanical properties of SA 508 and SS 304 LN steels (which respectively serve as the structural materials for RPV and calandria vessel) and Zr-2.5Nb alloy, which is the pressure tube material for commercial Pressurized Heavy Water Reactors. The analytical investigation of ABI technique for these materials is also presented.