The coating modification of polyacrylonitrile (PAN) fibers with boric acid to enhance the controllability of thermally oxidative stabilization process. The stabilization process, cross-section morphologies of oxidized and carbonized products were investigated by means of optical microscopy, SEM, XPS and in-situ thermal shrinkage indicator. The results indicated that the coating with boric acid on fiber surface was effective to avoid skin-core heterogeneity on the cross section and, in the stabilization process, that boric acid as a crosslinking agent to tie together the adjacent oxidative molecular chains was confirmed. It was suggested that the crosslinked structures should play an essential role in controlling the formation of uniform oxidized structures, which is favorable for tensile properties of carbon fibers.