Strain-induced crystallization which occurs at the crack tip is thought to be the origin of excellent strength of natural rubber (NR). However, the detailed mechanism of the reinforcement by the strain-induced crystallization is still unclear. By the X-ray diffraction mapping technique, we find that paraffine crystals are specifically formed around crack tip where strain-induced crystallization is observed in a carbon black filled NR sample. Dissipation of energy for the transportation of paraffine molecules from inside the sample to the surfaces is thought to partly contribute the reinforcement of the material.