Bombyx morisilk 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 Cβ peaks in the 13C cross-polarization/magic angle spinning NMR spectra of (Ala–Gly)m(m= 9, 12, 15, and 25) and 13C selectively labeled (Ala–Gly)15model peptides. Namely, three Ala Cβ peaks with relative intensities of 1:2:1 obtained by deconvolution were assigned to two kinds of β-sheet and a β-turn, which are interpreted as a lamellar structure formed by repetitive folding using β-turns every eighth amino acid, for which the basic structure is (Ala–Gly)4in an antipolar arrangement. The dynamics and intermolecular arrangement were further studied using 13C solid-state spin–lattice relaxation time observations and the rotational echo double resonance experiments, respectively.