Bombyx mori silk fibroin (SF) fibers with excellent mechanical properties have attracted widespread attention as new biomaterials. However, the structural details are still not conclusive. Here, we propose a lamellar structure for the crystalline domain of the SF fiber based on structural analyses of the Ala C beta peaks in the C-13 cross-polarization/magic angle spinning NMR spectra of (Ala-Gly)(m) (m = 9, 12, 15, and 25) and C-13 selectively labeled (Ala-Gly)(15) model peptides. Namely, three Ala C beta peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of beta-sheet and a beta-turn, which are interpreted as a lamellar structure formed by repetitive folding using beta-turns every eighth amino acid, for which the basic structure is (Ala-Gly)(4) in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using C-13 solid-state spin-lattice relaxation time observations and the rotational echo double resonance experiments, respectively.