Effect of network-chain density (v) on the strain-induced crystallization of peroxide-crosslinked natural rubber was studied by fast time-resolved synchrotron wide-angle X-ray diffraction. It was observed that the elongation ratio at the onset of strain-induced crystallization (alpha(c)) became smaller with the increase of v. The difference of the entropy between the undeformed and deformed states at each ac was nearly equal in spite of the variation of v. The calculated melting temperature at ac of the samples was also almost the same regardless of their v. These observations mean that the strain-induced crystallization occurred when the deformation brought about a definite entropic state for peroxide-crosslinked natural rubber. They agreed with the prediction by Flory and were consistent with the classical theory of rubber elasticity. (c) 2007 Elsevier Ltd. All rights reserved.