The active-to-passive oxidation transition for three types of Si-M-C-O fibers (Lox M, ZMI and SA) was examined at 1773 K and pO(2) = 1-25000 Pa through TG, XRD analysis, SEM observation and tensile tests. The pO(2) value of the active-to-passive oxidation transition for Lox M and ZMI fibers was between 100-250 Pa. While the strength of Lox M and ZMI fibers was considerably retained after passive-oxidation, it was markedly degraded after active-oxidation (sigma is approximately equal to 0 Gpa). For SA fibers, the passive-oxidation occurred even at pO(2) = 1 Pa, producing a very large decrease in the tensile strength.