The effects of polypropylene on the processing of low-oxygen silicon carbide (SiC) fibers by the polycarbosilane (PCS) route have been studied. Polypropylene acts as a high-temperature solvent for PCS, reducing the spinning temperature. A small amount of polypropylene (< 5 wt%) significantly improves the spinning ability and the tensile strength of the as-spun precursor fibers because of its excellent fiber-forming ability. During blending and spinning, no noticeable chemical interaction has been detected between PCS and polypropylene. In the early stage of electron beam irradiation of the polypropylene-containing precursor fibers, free radicals are formed from both PCS and polypropylene. They combine with each other, forming a cross-linked structure and promoting the curing. The introduction of polypropylene has no detrimental effects on the properties of the final SiC fibers.