The safety criteria of the fuel and the core of the High Temperature Engineering Test Reactor (HTTR) was examined in order to assess the safety of the reactor at anticipated operational transients and at accidents. The safety criteria at the transients were decided as follows; the maximum fuel temperature shall not exceed 1600degC for the reactor core fuel, and it shall not exceed 2500degC for the fuel limit irradiation specimen, which is a fuel assembly with a graphite block and fuel elements for studying the fuel failure limit. On the other hand, the criteria for the accidents were decided so that (i) the fuel rod shall be held within the graphite block, and (ii) the support post and the post sheet shall maintain the strength enough to support the reactor core. Validity of the above criteria was ensured by the fuel behavior at representative abnormal events, namely, `abnormal events at the fuel limit irradiation test`, `rapid reactivity insertion events`, `air ingress accidents`, and `fuel assembly fall accidents`. (author).