A series of high-resolution solid-state C-13 NMR experiments were performed on both unstretched and in situ stretched natural rubber samples. From the C-13 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 C-13 DD/MAS spectra, the crystalline signals were found to increase with the increase of draw ratio. C-13 spin-lattice relaxation time (T-1) and H-1 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 C-13 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. (C) 2004 Elsevier Ltd. All rights reserved.