Digital systems are used in nuclear facilities to monitor and control various types of field devices, as well as to obtain and store vital information. Therefore, it is getting important for nuclear facilities to protect digital systems from cyber-attack. Generally, critical digital assets are estimated over 70% of all digital assets in a nuclear power plant. Therefore, it is required to prioritize critical digital assets to improve an efficiency of regulation and implementation. A regulatory body and nuclear licensees will be able to more focus on vital digital assets which are based on the graded approach for cyber security. In this study, we developed a methodology identifying vital digital assets (VDAs) based on PSA (Probabilistic Safety Assessment) and applied the method to a nuclear power plant model. At this point, only mitigating systems except IEs (Initiating Events) were considered to identify the VDAs since IEs are not fully developed as fault tree models. We analyze whether the failure of event mitigation can be caused by a failure of digital assets due to cyber-attack. The analysis of the MCS (minimal cut set), which can induce reactor core damage has already been carried out for a long time in the PSA model, and the results are used in this study. The process of identifying VDAs that prevent a failure of accident mitigation is as follows; 1) Mapping events to cabinet failures, 2) replace events with cabinet failures, 3) Calculate MCSs (target sets) consisting of combination of digital assets (cabinets) through an event tree (ET) and a fault tree (FT) analysis for the corresponding initiating event, 4) Calculate prevention sets (digital assets sets), 5) Select the most effective prevention set which can be VDAs. Through this study, we have conceptually confirmed that the failure of controlling digital assets due to cyber-attack affect the system and can affect initiating events of nuclear power plants and failure of accident mitigation. The VDAs are defined as sets of digital assets that can prevent failure of mitigation for accidents inducing a core damage of a nuclear power plant. This method was applied in a full power PSA model for a nuclear power plant and sets of VDAs were derived.