Insoluble sulfur (IS), a long chain of sulfur, which is prepared by polymerization of cyclooctasulfur (S8) via thermal ring opening. As IS is very important as a vulcanizing agent for rubber, it is worth enhancing the thermal stability of IS and the yields from S8 for its actual industrial usage such as tire manufacturing processes. Post heat-treatments are effective in the enhancements, but the mechanism remains unclear. This is because the insolubility of IS limits the methods available for characterization. Electron spin resonance (ESR) spectroscopy can detect and identify chemical species having unpaired electrons with high sensitivity. It is widely used to elucidate polymerization and degradation mechanisms of rubber and plastics. IS has radicals at both ends of its linear chain. In this review, we focus on sulfur radicals to characterize IS using the ESR technique and discuss reactivities of sulfur radicals in the polymerization of sulfur via S-S bond cleavage of S8, the evaluation of sulfur chain length, and the dependence of stability of IS on post heat-treatments in terms of the chain length.