Although there are lots of efforts for human operators to reduce human errors in systematic and managerial ways in nuclear power plants, human operators still encounter unexpected plant conditions. Since unexpected plant conditions require a human operator to do very different information processing with anticipated plant conditions and enable wide range of error mechanisms and error types, so it is necessary to understand cognitive process itself for appropriate situation assessment. Second generation human reliability analysis methods, cognitive models, and quantitative models for situation assessment have been developed for a human operator to understand cognition process and to estimate situation assessment, but they do not include the effects of unexpected plant conditions with an explicit set of influencing factors. This paper suggests influencing factors on situation assessment of an individual human operator in unexpected plant conditions. A flow chart was developed for an inductive approach from the existing HRA methods and cognitive models. As a result, five (5) situational factors "procedure", "complexity", "available time", experience/training", "threat" and two (2) personal factors "stress", "personality (individual characteristics)" were identified as influencing factors on situation assessment of an individual human operator in unexpected plant conditions. In order to explain how the influencing factors affect human cognition in unexpected plant conditions, a cognitive model was introduced by applying the influencing factors. The influencing factors and its cognitive model will be used to quantify the effects of unexpected plant conditions on situation assessment of an individual human operator. (C) 2019 Elsevier Ltd. All rights reserved.