Abstract: Environmental effects, especially O₂ pressure, often make SiC/SiC composites degraded. In order to investigate the effects on microstructural stability, SiC/SiC composite specimens, made of Hi-Nicalon^TM SiC fibers, were annealed under a commercial grade Ar flow and an ultra-high purified Ar flow respectively. The microstructural evolution in SiC/SiC composites was examined by a field emission type high-resolution electron microscopy and a parallel electron energy loss spectroscopy. A possible mechanism for this degradation was discussed.