Polycarbosilane (PCS) is the precursor of SiC ceramic fibers

and oxidation-curing process of PCS fibers is an essential step of making Nicalon style SiC fibers．The oxygen introduced into PCS fibers seriously affects the mechanical proporties of SiC fibers at high temperature．In this paper

the probability whether PCS fibers can be thermal-cured without oxygen was investigated．It was found that a minimal degree of oxidation-curing of PCS fibers was necessary for their further thermal curing Under these conditions the oxygen content of fibers decreases

and their pmperties can be improved．The mechanism of thermal-curing of low degree oxidation-curing PCS fibers and the function of oxygen were investigated Results showed that the minimal degree of oxidation-curing was corresponding to a 9％ weight gain of PCS fibers．During the thermal-curing process

the Si-H of bonds PCS were mostly consumed and Si—CH2一Si structure Was formed；Si一OH was formed when a little oxygen was introduced into PCS fibers in the oxidation-curing process and may form Si—O—Si structure during further thermal-curing

making a protective layer on the surface of PCS fibers．