The dynamic mechanical and thermal properties of natural rubber/poly (methyl methacrylate) blends (NR/PMMA) with and without the addition of graft copolymer (NR-g-PMMA) have been investigated. Dynamic mechanical spectroscopy is used to examine the effect of compatibilizer loading on storage modulus (E'), loss modulus (E ") and loss tangent (tan delta) at different temperatures and at different frequencies. The morphology of the blends indicates that the size of the dispersed phase decreased by the addition of a few percent of the graft copolymer followed by a leveling off at higher concentrations. This is an indication of interfacial saturation. Attempts have been made to correlate morphology with dynamic mechanical properties. Various models have been used to fit the experimental viscoelastic results. Differential scanning calorimetry has been used to analyze the glass-transition temperatures of the blends. The thermal stability of the blends has been analyzed by thermogravimetry. Compatibilized blends are found to be more thermally stable than uncompatibilized blends. Finally the miscibility and mechanical properties of the blends annealed above T-g are evaluated.