The active-to-passive oxidation transition for three types of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon and Hi-Nicalon S) was examined at 1773 K through TG, XRD analysis, SEM observation and tensile tests. The oxygen partial pressure for the active-to-passive oxidation transition decreased in the following order: p(O2) = 100-250 Pa for Nicalon, p(O2) = 10-25 Pa for Hi-Nicalon and p(O2) = 1-2.5 Pa for Hi-Nicalon S. Considerable strength was retained in the passive-oxidation region. The active-oxidation produced a marked decrease in strength of Nicalon and Hi-Nicalon (sigma is approximately equal to 0 GPa). On the other hand, the strength of Hi-Nicalon S after active-oxidation was nearly identical to that after passive-oxidation (sigma is approximately equal to 1 GPa).