The fracture behavior of the Tyranno-ZMI fiber exposed in air at 1173 ∼ 1673K for 2×104s was studied. The average strength of the fiber with SiO2 layer on its surface decreased and the scatter of strength increased with increasing exposure temperature. The fracture of the fiber was controlled by the propagation of the crack made by the premature fracture of the SiO2 layer into fiber. The average strength of the fiber without SiO2 layer prepared by etching away the SiO2 layer from the fiber exposed in air, decreased also with increasing exposure temperature, whereas it was far higher than that of the fiber with SiO 2 layer. The fracture toughness of the fiber, which was originally amorphous decreased with increasing exposure temperature due to the crystallization. In this way, the high temperature-exposure in air caused degradation of the fiber itself and the SiO2 layer-originated crack propagation.