ABSTRACT:The dynamic mechanical properties of macro and microfibers of oil palm-reinforced natural rubber (NR) composites were investigated as a function of fiber content, temperature, treatment, and frequency. By the incorporation of macrofiber to NR, the storage modulus (E') value increases while the damping factor (tan d) shifts toward higher temperature region. As the fiber content increases the damping nature of the composite decreases because of the increased stiffness imparted by the natural fibers. By using the steam explosion method, the microfibrils were separated from the oil palm fibers. These fibers were subjected to treatments such as mercerization, benzoylation, and silane treatment. Resorcinolhexamethylenetetramine-hydrated silica was also used as bonding agent to increase the fiber/matrix adhesion. The storage modulus value of untreated and treated microfibril-reinforced composites was higher than that of macrofiber-reinforced composites. The T g value obtained for this microfibril-reinforced composites were slightly higher than that of macrofiber-reinforced composites. The activation energy for the relaxation processes in different composites was also calculated. The morphological studies using scanning electron microscopy of tensile fracture surfaces of treated and untreated composites indicated better fiber/ matrix adhesion in the case of treated microfibrilreinforced composites. Finally, attempts were made to correlate the experimental dynamic properties with the theoretical predictions.