In a previous work [Macromolecules 2005, 38, 3783], nanocomposite materials obtained from a latex of natural rubber (NR) as matrix and an aqueous suspension of waxy maize starch nanocrystals as filler were processed and characterized in terms of morphology, structural, and barrier properties. The aim of the present work was to study the mechanical properties of these materials in both the linear and nonlinear range. It was shown that up to a content of 20 wt % this new filler presents the advantage to reinforce NR without decreasing significantly the elongation at break of the material. The relaxed modulus at room temperature of nanocomposite films containing 20 wt % of filler is 75 times higher than the one of the unfilled matrix. Particle-particle interactions play a crucial role in the properties of starch nanocrystals filled NR. The effect of moisture content and surface chemical modification of the starch nanocrystals on the reinforcing properties was also investigated.