The structural analysis of Bombyx mori silk fibroin (SF) has received considerable attention because of its supreme physical and medical properties. These properties originate from the structure of silk fibroin, making structural analysis key to the further development of silk in wider applications. Two distinct structures of SF in the solid state have been reported, i.e., Silk I as the solid-state SF structure stored in the middle silk glands after drying without any external forces, and Silk II as the structure after spinning. Here, detailed structures of Silk I and Silk II were determined using mainly solid-state NMR. Silk I was proposed to be a repeated type II β-turn structure with the torsion angles (φ, ψ) = (−62°, 125°) for Ala residue and (φ, ψ) = (77°, 10°) for Gly residue. The intra- and intermolecular hydrogen bonding was formed alternatively along the chain. On the other hand, Silk II was proposed to be a lamella structure, i.e., two kinds of Ala residues in β-sheet structure, plus one distorted β-turn formed by repetitive folding using β-turns every eighth amino acid in an antipolar arrangement.