The natural rubber (NR) nanocomposites reinforced with nanodiamond (ND) were prepared by a simple casting method. The structure and properties of the nanocomposites were investigated. ND consists with the core of diamond primary particle and the shell of graphite-like layer with oxygen containing groups. Due to the core, ND shows excellent properties derive from diamond. The oxygen containing groups on the surface enables ND to nano-disperse in water. In addition, these groups can be sites of chemical modification. In the present study, in order to generate strong interfacial interaction between NR and ND, chemical modification of ND was conducted using octadecylamine and sulfur. The high dispersibility of the fillers in NR were revealed by scanning electron microscopy. The cross-linking density of NR was increased by NDs, especially by modified NDs. Due to the strong interaction at the interface between NR and fillers, the nanocomposites showed excellent mechanical properties showing that prominent reinforcement effect of ND was successfully imparted to the nanocomposites.