Polycarbosilane (PCS) precursor fibers were irradiated by γ-rays in air. The structure and the subsequent pyrolysis reaction of PCS fibers irradiated below gelling dose were investigated. Results show that a little of oxygen is imported to form the structures of Si-O-Si、Si-OH and C=O from Si-H and Si-CH,3, of PCS in irradiation process. And gelatinized products formed from the Si-O-Si and Si-C-Si bridge structures obviously increase with the heat treatment temperature. It has been also found that the initial pyrolysis temperature of irradiated PCS fibers increases from 320℃ to 600℃, and the ceramic yields increase from 57.5% to about 83%. All the samples of PCS precursor fibers irradiated below gelling dos and under the condition with a little of oxygen can be pyrolyzed into SiC ceramic fibers with intact microstructure.