Montmorillonite nanoclay was modified with octadecylamine and compounded with natural rubber by the melt mixing method. Influence of the modification on the properties of natural rubber/organoclay nanocomposites was investigated. Furthermore, structures of the natural rubber/clay nanocomposites were characterized by X-ray diffraction and scanning electron microscopy. It was found that organoclay was well dispersed in the natural rubber matrix. Curing characteristics of the rubber compound indicated that octadecylamine modification accelerated the vulcanization reaction of natural rubber and caused a higher degree of crosslinking. The storage modulus (E') of the natural rubber vulcanizates was higher with the modified clay than with unmodified montmorillonite (Na-MMT). The effects on the storage modulus are more pronounced at temperatures above the melting temperature of octadecylamine. The low thermal stability of this modifying agent does not affect the thermal stability of the natural rubber/organoclay nanocomposite.