Natural rubber(NR)/graphene(GE) composites were prepared by an ultrasonically-assisted latex mixing and in-situ reduction process. Graphene oxide was dispersed in natural rubber latex using an ultrasonic field and then was in-situ reduced with hydrazine hydrate, followed by latex coagulation to obtain the NR/GE masterbatch. Through further dilution of the masterbatch with normal NR by a twin-roll mixing process, NR/GE composites with different GE contents were obtained. The results show that the process produces a much better dispersion and exfoliation of GE in the matrix and contributes to an increase in the tensile strength compared to conventional direct mixing. Compared to pure rubber, the tensile strength and tear strength for NR/(2 wt%)GE composites were increased by ~47% and ~50%, respectively. Among three different carbonaceous fillers, carbon black(CB), multi-walled carbon nanotubes(MWCNT) functionalised with hydroxyl groups and GE, GE is the best to reinforce NR. The stress at 300% strain for NR/(2 wt%)GE composites is respectively 2 and 2.5 times that for NR/(2 wt%)MWCNT and NR/(2 wt%)CB composites. An improved strain-induced crystallization effect during stretching was observed for NR/GE composites.