In the present work, the influence of graphene (GE) on the vulcanization kinetics of natural rubber (NR) with sulfur curing system was investigated in detail for the first time. It is found that on adding graphene the induction period of the vulcanization process is remarkably depressed, whereas the vulcanization rate is enhanced at low graphene loading and then suppressed. As a result, the optimum cure time decreases dramatically at first and subsequently shows a slight increase with increasing graphene loading. At the same time, the crosslinking density of NR increases monotonically, because graphene takes part in the vulcanization process. The exothermal peak of the vulcanization reactions is split into two peaks on adding >= 0.5 phr graphene. It is interpreted in terms of two reaction stages, i.e., chemical reaction controlling stage and diffusion controlling stage. The activation energy of the former stage decreases with increasing graphene loading, while that of the latter stage is higher than the former one and increases with graphene loading. A possible mechanism was proposed to interpret the accelerating effect of graphene and the enhanced crosslinking density of NR. (C) 2013 Elsevier Ltd. All rights reserved.