In the lifecycle of a nuclear power plant (NPP), commissioning is the last stage before commercial operation. To understand the nature of human errors in NPP commissioning tasks, the IDHEAS (Integrated Human Event Analysis System) method was adopted in this study to analyze 311 incident reports of NPP commissioning tasks, i.e. identifying the human errors as well as macrocognitive functions, proximate causes and cognitive mechanisms behind them. Results indicate that, in NPP commissioning tasks, individual failures of high-level mental processing had the highest occurrence frequency (59% of human errors with macrocognitive functions identified clearly), which should be specially concerned in risk management. As for human errors related with teamwork, failures of team communication were common in all categories of commissioning. The ratio of U1 items (reports didn’t provide sufficient information to analyze, 44% in total) emphasizes the necessity of a standardized format for incident reports to support HRA research. In this case study, analysis results presented an unsatisfactory between-analyst reliability of IDHEAS, and as a side product, four usability problems of IDHEAS were identified, indicating that some modifications are in need to improve the method.