Strain induced crystallization (SIC) of peroxide vulcanized natural rubber (NR)/ground tire rubber (GTR) composites is studied by combining mechanical and microstructural characterization techniques. It is found that the incorporation of GTR into the NR matrix leads to more effective reinforcement at large strains at room temperature in the NR/GTR composites as compared to the neat NR. It is attributed to (i) the presence of the GTR particles acting as reinforcing fillers owing their high carbon black content, and (ii) the nucleating effect of the GTR for SIC in the NR matrix inducing further reinforcement through the generation of an increasing number of oriented crystallites that behave as supplementary reinforcing fillers.