Commissioning is the last stage before operation of a nuclear power plant, aiming at validating all equipment and systems. The safety and reliability of commissioning tasks can be severely threatened by human failures. Therefore, it is essential to conduct causation analysis of human failure events in commissioning to identify salient human and organizational factors. The human factors analysis and classification system (HFACS) framework was modified and applied to code 127 human failure events in commissioning by five domain experts. The comparison between the results of this study and the analyses of operation events indicates that commissioning and operation as two different stages of a nuclear power plant have similar patterns in regard to supervision and management and human error types, but distinct patterns in regard to organization, preconditions of unsafe acts, and human violations. Results of correlation analysis show that several preconditions for unsafe acts have significant relations with human failures, as well as some significant relations between latent failures. These human and organizational factors should be controlled to reduce human failures.