Abstract: Polycarbosilane (PCS) precursor fibers were irradiated by γ-ray in air. Silicon nitride ceramic fibers were obtained from the irradiated PCS fibers by process of thermal cross-linking treatment in Ar, ammoniation pyrolysis in NH₃ and high temperature nitriding treatment in N₂. The effects of thermal cross-linking treatment on chemical structure, gel content, ammoniation ceramic yield, tensile strength, microstructure and oxygen content were studied. The results show that bridge structures of Si—CH₂—Si and Si—O—Si are formed in the PCS fibers by thermal cross-linking treatment and make the PCS fibers become gelling. The ammoniation ceramic yield of the irradiated PCS fibers increases sharply after thermal cross-linking treatment. The tensile strength of the silicon nitride ceramic fibers increases sharply, while the oxygen content decreases. After thermal cross-linking treatment, the tensile strength of the silicon nitride ceramic fibers derived from the PCS fibers with absorbed dose of 1.0 MGy reaches 2.05 GPa and the oxygen mass fraction is 9.5%.