A series of high-resolution solid-state 1 3 C NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the 1 3 C CP/MAS spectra, it was found that natural rubber does form small crystals at room temperature though the degree of crystallinity is very small. Furthermore, from the 1 3 C DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. 1 3 C spin-lattice relaxation time (T 1 ) and 1 H spin-spin relaxation time (T 2 ) of in situ stretched natural rubber were measured for the first time, which provided further evidences for the conclusion that there exist crystals in both stretched and unstretched natural rubber samples. Quantitative 1 3 C NMR measurements indicated that strain-induced crystallization occurs when the draw ratio reaches about 2.0 and the maximum crystallinity of our natural rubber samples can be as high as 19.3% upon stretching.