We suggest a model that describes the observed non-monotonous temperature dependences of the flexural strength of ceramics. Within the model, the flexural strength is affected by the sliding of the intergranular boundaries, which can blunt the crack tip and increase the flexural strength at certain temperatures. At the same time, at high enough temperatures, enhanced boundary sliding results in the transition from the brittle to ductile failure, which reduces the flexural strength. It is demonstrated that the fracture strength of ceramics at elevated temperatures can be strongly affected by the sliding properties of the intergranular boundaries and the loading time. The ceramics with the highest fracture strength should have low sliding resistance at short-term loading and high sliding resistance in the case of long-term loading. The results of the model quantitatively agree with experimental data.