Polycarbosilane (PCS) fiber was irradiated by electron beam at low dose in a flowing N-2/O-2 mixture with O-2 concentration of 1%. After the pyrolysis of the irradiated precursor fibers, SiC fibers with high strength of 2.4GPa were obtained. Microstructural evolutions of the resultant fibers were explored. It was found that during the irradiation, free radicalswere formed in the PCS and were oxidized by oxygen as Si-OH groups. The Si-OH groups then transformed into Si-O-Si linkage and resulted in further cross-linking of the PCS during pyrolysis. A remarkable structure gradient along the fiber diameter was formed under the coupled effects of irradiation and oxidation. The content of oxygen decreased from the fiber surface to the core, whereas the crystallinity of -SiC increased in the same direction. The electrical resistivity of the as-prepared ceramic fiber was 80.7cm, showing good potential for being as electromagnetic wave absorber.