Polycarbosilane (PCS) ceramic precursor fibers are irradiated in a nuclear reactor and pyrolyzed under inert atmosphere. Bridge structure of Si-CH2-Si is formed in the irradiated products by the rupture of Si-H bonds and succeeding cross-linking. When irradiated at the neutron fluence of 2.2 x 10(17) cm(-2) under N-2 atmosphere, the gel content and ceramic yield at 1,273 K of PCS fibers are up to 80% and 94.3%, respectively, and their pyrolysis products are still fibrous, which illuminates that the infusibility of PCS fibers has been achieved. FT-IR spectra indicate that the chemical structure of pyrolysis products is very similar to that of pure SiC, while X-ray diffraction curves suggest that beta-SiC microcrystals are formed in the fibers, and their mean grain size is about 7.5 nm. The oxygen content (1.69-3.77 wt%) is much lower than that of conventional SiC fibers by oxidation curing method (about 15 wt%). Tensile strength of the SiC fibers is up to 2.72 GPa, which demonstrates that their mechanical properties are excellent. After heat-treated at 1,673 K in air for an hour or at 1,873 K under Ar gas atmosphere for 0.5 h, their external appearance is still undamaged and dense, and their tensile strength decreases to a small extent, which verifies that heat resistance of the SiC fibers is eximious.