Abstract: Catalysts play a key role in the manufacture of rayon-based carbon fibers. The pyrolysis behavior of rayon fibers treated with boron (B), phosphorous-nitrogen (PN) and boron-phosphorous-nitrogen (PNB) containing catalysts were investigated by thermogravimetry- mass spectrometry (TG-MS) analysis in an inert atmosphere. It is found that the addition of B, PN and PNB resulted in a lower pyrolysis temperature and an increase of char, water and carbon dioxide at 600℃. PN and PNB restrained the formation of tar, aldehyde derivatives and furan derivatives whereas B did not. The catalysis of B and PN used together is more effective than the single use of B or PN because of their cooperative effect on the pyrolysis of rayon fiber.