Electron spin resonance (ESR) measurements under the tensile deformation of cross-linked polyisoprene were examined by the combination of home-built stretching machine with commercially available ESR spectrometer. The changes of radical concentration during tensile deformation were successfully determined by this technique. It was found that the number of radicals produced by the stretching process decreased during the retraction of stretched sample. In addition, at a given strain, the decrease of radical concentration with time was accompanied by the increase of stress. Such changes were more prominent in the cross-linked sample by Sulfur PS sample (PS) than by dicumyl peroxide (PCP), PD sample (PD). The tensile modulus of the samples after the one cyclic deformation increased with increasing the keeping time at room temperature after the cyclic deformation. The increase was larger for the PS than for the PD. These results Suggest that the broken cross-linking structure induced by the stretching process was partly reconstructed during and after the retraction with this tendency more prominent in the PS than in the PD. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2791-2795, 2009