The purpose of this paper is to clarify the role of crosslink density of natural rubber on its strain-induced crystallization. A series of new in situ synchrotron X-ray diffraction experiments were performed during the stretching process of weakly and highly vulcanized natural rubber samples. The experimental data have been analysed in terms of both crystallite size and crystallization rate. Moreover, a careful treatment of previously published data that might appear contradictory has been done. The comparison between all these data, coming from NR of different origins and with different crosslinking states, demonstrates that the molecular weight between physical entanglements in natural rubber appears as a key parameter for strain-induced crystallization (SIC). (c) 2006 Elsevier Ltd. All rights reserved.