In this study, SiCf/SiC composite materials with PyC/(SiC)4 multilayer interfacial phase were subjected to thermal treatments at 1050 and 1350 ℃ in argon and air media for durations of 0.25, 1, 4 and 16 h. The microstructural evolution and the relationship with mechanical properties after different high-temperature media thermal treatments were analyzed. The results showed that there was no change in the microstructure of the composite material after exposure to high-temperature argon media. However, with the increasing in treatment time and temperature, the tensile strength of the material decreased due to thermal damage of the fibers and matrix, while the composite still maintained the toughness characteristics. In the air media, after short-term oxidation at high temperatures, oxidation loss occurred at the PyC interface of the composite material, resulting in annular voids between fibers and SiC interfacial phases. After long-term oxidation at high temperatures, these annular voids were filled with SiO2. These changes in microstructure resulted in three main tensile fracture morphologies for SiCf/SiC composites: materials contained PyC interfaces had moderate fiber pull-out lengths, materials without PyC interfaces had longer fiber pull-out lengths, while materials with a layer of SiO2 had no fiber pull-out observed. The overall tensile strength of SiCf/SiC composites treated in high-temperature air media was lower than that treated with argon media, mainly due to oxidative damage to fibers and matrix and loss of toughness caused by formation of SiO2 at interfaces.