To clarify the effects of reduced pressure on the thermal stability of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon and Hi-Nicalon S), the heat-treatments were conducted at 1623 and 1723 K under total pressures (pT) of 1 - 10(5) Pa. The oxidation behavior and thermal stability of the fibers were investigated through examinations of mass change, grain growth, specific resistivity, fiber morphology and tensile strength. All the fibers were definitely oxidized in the active-oxidation regime at p(T) less than or equal to 10(2) Pa and T = 1723 K. Nicalon and Hi-Nicalon S were subjected to serious degradation of fiber strength. Hi-Nicalon had a strength of 1.2 GPa even after heat-treatment at p(T) = 1 Pa. (C) 2003 Kluwer Academic Publishers.