It is expected that in the future SiC fiber-reinforced ceramic-matrix composites (CMCs) will be used in high temperature and hostile environments. In this study, Hi-Nicalon(TM) SiC fibers were annealed and crept at 1500 degrees C for 1 hour in air, an argon flow and an ultra high-purity argon flow in order to investigate the effects of atmospheres and load conditions on the decomposition behavior and microstructural evolution of the fibers. After the fibers were annealed and crept in air, a silica layer with cracks was formed on the fiber surface. Under the creep load, the silica layer became thicker and porous due to the oxidation mechanism change from diffusion of ionic oxygen to transportation of oxygen molecules. An oxygen-enriched amorphous layer was formed at the fiber surface in the case of annealing in an argon flow, whereas SiC crystals were produced by the gas-phase reaction on the fiber surface when the fiber was crept in an argon flow. In an ultra high-purity argon flow, SiC crystals grew on the surface of both annealed and crept fibers. Growth of beta-SiC grain was enhanced under low oxygen partial pressure atmospheres and creep load.