In this paper, the synergistic effects of temperature and time on the proportional limit stress (PLS) of SiC/SiC composites are investigated. The temperature-dependent constituent properties and time-dependent interface oxidation are considered to determine the PLS. The effects of fiber volume fraction, interface shear stress, interface debonded energy and matrix fracture energy on the temperature/time-dependent PLS and interface debonded length of SiC/SiC composite are discussed. The experimental PLS of SiC/SiC composite corresponding to different temperature and time are predicted. [GRAPHICS] .