Amorphous silicon nitride fibers were characterized with respect to their chemical and phase compositions. The high-temperature behavior of the fibers was investigated in N-2 and Ar at temperatures ranging from 1350 to 1600 A degrees C through tensile tests, pore volume analysis, elemental analysis, weight loss, X-ray diffraction, and scanning electron microscopy to elucidate the effects of the atmosphere on the fiber degradation mechanism. The fibers demonstrated better high-temperature stability in N-2 instead of Ar. In particular, the fibers heat-treated in N-2 at 1450 A degrees C retained 43% of their original strength (0.64 GPa), while the fiber strength was completely lost in Ar at the same temperature. The primary reason for the strength degradation of the amorphous fibers was the expanding nanopores before the formation of the large Si3N4 grains. However, a catastrophic decrease in strength was due to the drastic thermal decomposition of SiNxOy and crystallization of Si3N4 grains, influenced by N-2 pressure. Thermodynamic considerations were additionally applied to estimate the decomposition and carbothermal decomposition of crystalline Si3N4, thereby explaining the significant discrepancy of crystallization transformation at 1600 A degrees C under different atmospheres.