IAEA initiated safety concept of Defense-in Depth (DiD) for nuclear power plants (NPPs) has been promoted in all the nuclear developing countries around the world. Thus far, the authors of this paper have been developing various risk analysis methods and conducting the application studies to be applicable for any layers of DiD of NPPs. In this paper, a unified methodology of “integrated DiD risk analysis system” which is composed by three core systems, that is, (i) DiD risk state estimation and scenario generator, (ii)plant DiD risk monitor, and (iii)reliability monitor. Those three core systems will utilize many supportive toolkits for each layer of DiD to generate appropriate software system of either (i)offline analysis for designing and evaluation of a specific application, or (ii)online real-time processing for HIS (human system interface) for monitoring, diagnosis and procedure presentation. The basic idea of why, what and how to integrate and where to be used are overviewed prior to the software system development for individual purpose of usage either off-line or on-line.