Strong reinforcing effects are observed when multiwall carbon nanotubes are incorporated in a styrene-butadiene copolymer. The good level of dispersion of carbon nanotubes in the host matrix that allows to take advantage of their high aspect ratio as well as their orienting capability under uniaxial extension, appear to be the main parameters for mechanical reinforcement. Atomic force microscopy carried out in the unstretched and stretched states has been shown to bring important insights for the understanding of the mechanical properties of the composites.