As an advanced in-core material in high temperature gas-cooled reactors (HTGRs), superplastic ceramics is attractive due to the possibility of the plastic working. For the application to the nuclear fields, the basic concept of design criteria was studied for typical superplastic ceramics, tetragonal zirconia polycrystals containing 3mol% yttria (3Y-TZP). The experimental results on 3Y-TZP showed that it is possible to apply the Weibull weakest-link theory to decide the stress limits in the criteria. The Weibull parameter m was evaluated as 9.5 for the bending and as 26.5 for the compressive. The applicability of the Weibull theory was also verified by the bending test results with different span. Based on the graphite structural design guidelines for the High Temperature Engineering Test Reactor (HTTR), the design stress limits for 3Y-TZP was proposed. It was shown that the proposed stress limits have appropriate safety margin and thought to be effective to evaluate the integrity of in-core structure made of 3Y-TZP.