A better understanding of the correlation between the primary sequences and the confOrmations of silk fibroiris (SF) is necessary in order to use silk more effectively as a functional material. In this paper, C-13 CP/MAS NMR was used to monitor separately the conformational transitions of [3-C-13]Ser-, [3-C-13]Tyr-, and [3-C-13]Ala-labeled Samia cynthia ricini SF induced by stretching. The conformation was mainly alpha-helix with no beta-sheet structure prior to stretching. At a stretching ratio of xS, three peaks assigned to beta-sheet structure were observed, and the fraction of these peaks increased rapidly upon further stretching. In particular, a rapid increase of the fraction of beta-sheet at more than XS stretch was observed for the Ser residues that were associated with the transition of the polyalanine chain. For the Tyr residues, the transition from random coil to beta-sheet occurs independently of the transition of the crystalline domain. These correlations were quantified and may be useful in future designs of artificial silk structure.