The vulcanization mechanism remains unclear due to the difficulty of characterization and the complexity of the vulcanization systems. In this work, a series of epoxy model systems were used to study the sulfur cleavage behavior, from which the study process of the vulcanization mechanism was significantly simplified and a brand new sulfur-only vulcanization mechanism was revealed by the FTIR and DSC analysis. Our studies suggest that thiyl radicals derived from the homolytic cleavage of sulfur play the dominant role in the vulcanization, while the heterolytic fission of sulfur cannot occur as expected because this reaction needs 200 degrees C. Nevertheless, sulfur anions, which are easy to be understood derived from the heterolytic cleavage of sulfur, are actually transformed from the thiol groups that are produced through the reaction of thiyl radicals abstracting alpha-H atoms of allyl groups in rubber, and this maybe the reason that vulcanization mechanism remains controversial for centuries.