Peroxide induced crosslinking and degradation of nylon-12 were experimentally investigated using an on-line electron spin resonance (ESR) spectrometer. It was found that different types of polymer radicals were involved in various peroxide modification processes. Five-line ESR spectra were observed in the DCP/nylon-12 and TBPB/nylon-12 systems, while the L130/nylon-12 system gave a three-line spectrum initially, followed by a transition to a single line. Based on the radical information, as well as gelation and viscosity measurements, various reaction mechanisms were proposed depending on the type of peroxide. TBPB is in favor of the degradation of nylon-12, while DCP and L130 promote crosslinking of the polymer. The gel formation was followed to facilitate the study of crosslinking kinetics. The initial gelation rate increased linearly with the peroxide concentration and/or temperature. The formation of crosslinked network impeded the crystallization of nylon-12 and reduced the degree of crystallinity. The high crosslinking extent resulted in low melting temperature and heat.