The thermal and mechanical stability of SiC fibers at elevated temperature is an important property for the practical application of SiC fiber-reinforced ceramic matrix composites and is related to the heat-treating atmosphere. In this study, the high-temperature behavior of KD SiC fibers with low oxygen content was investigated in both Ar and N-2 at temperatures from 1400 to 1800 A degrees C through scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Auger electron spectroscopy, resistivity measurements, and tensile tests in order to understand the effects of atmospheres on the degradation of the fibers. The results show that high-temperature treatment caused more severe strength degradation in Ar than in N-2. In particular, the fibers heat treated in N-2 at 1700 A degrees C retained a relatively high strength of 1.52 GPa, 60 % of their original strength, while the fiber strength was completely lost after heat treatment in Ar. Fiber strength degradation was mainly caused by a combination of crystal growth and surface flaws. The formation of huge grains and porosity in the fiber surfaces, owing to the thermal decomposition of the SiC (x) O (y) N (z) and SiC (x) O (y) phases, significantly degraded the strength for fibers heat treated in Ar. However, the suppressing effect of N-2 on the decomposition of the SiC (x) O (y) N (z) phase in the fiber surfaces and nitrided case on the decomposition of the SiC (x) O (y) phase in the fiber cores, led to higher SiC fiber temperature stability in N-2 rather than Ar.