A modular high-temperature gas-cooled reactor (MHTGR) is expected to be one of the best energy sources in the near future because it can supply high-temperature heat and have high thermal efficiency and sufficient safety features. The safety evaluation of the future MHTGR should be performed based on the experience obtained from the safety evaluation of the High-Temperature Engineering Test Reactor (HTTR). The safety evaluation of the HTTR was performed considering the specific safety design features of the HTGR and is applicable to the future MHTGR. Before the detailed safety evaluation of the future MHTGR, the safety evaluation method and results of the HTTR should be reviewed, and newly established acceptance criteria and methods for selecting evaluation events must be clarified. This paper describes in detail the method and results of the safety evaluation of the HTTR.