Abstract It has been expected that the automation of certain tasks in a control room would help decrease operators’ mental workload, enhance situation awareness, and improve the whole system performance. However, there have been too many automation-induced system failures that would warrant a fresh look on the influences of automation. Automation problems include the reduction in the operator's system awareness, an increase in monitoring workload, and the degradation in manual skills. This study evaluates operators’ mental workload and system performance during a human–system interface (HSI) automation in an advanced nuclear power plant (NPP). The reactor shutdown task and alarm reset task simulations were conducted in this study to evaluate operators’ mental workload and performance. The results of this study indicated that for ensuring safe operating in NPPs, the design of automation needs to be carefully implemented. Task characteristics and degrees of automation should be carefully evaluated while designing HSIs. The reactor shutdown tasks studied in this paper suggest that a high level of automation design for the long period and low workload would be sufficient. On the other hand, the degree of automation of alarm reset task does not show a significant difference to the operator's mental workload. In conclusion, the human–system interface automation in advanced NPPs is suggested to be more flexible and needs to be continually improved.