Variation of crosslink density of NR vulcanizates by thermal aging at room temperature and at constant temperatures of 40 and 80℃ with the aging time was studied. Nine NR vulcanizates with various cure systems (conventional, semi-EV, and EV) and cure times (under-, optimum-, and overcure) were prepared to investigate the influence of the initial crosslink type and degree of crosslink density on the variation of crosslink density of th vulcanizates. It was found that the crosslink densities of the nine vulcanizates were changed even at room temperatures as well as 40 and 80℃. The crosslink densities of the vulcanizates after the thermal aging at 80℃ for 10 days increased continuously, irrespective of the cure conditions, but those after the thermal aging at 40℃ decreased except only the undercured vulcanizate with the conventional cure system. Crosslink density of the undercured vulcanizate was changed more easily than those of the optimum-and overcured ones. Variation of the crosslink density of the vulcanizate with a high sulfur content after the thermal aging was larger than that with a low sulfur content. Degree of the crosslink density change of the elongated vulcanizate by the thermal aging was larger than that of the unelongated one.