Isotactic-poly(1-butene) (iPB1) shows superior mechanical properties after crystal–crystal transitions. Recently, Miyoshi et al. found that crystalline stems in metastable tetragonal crystal perform uniaxial rotational diffusions accompanying side-chain conformational transitions in the fast motional limit (correlation time, 〈τ_c〉 <10⁻⁷ s; Macromolecules 2010, 43, 3986–3989.). In this study, molecular dynamics in stable trigonal crystal is investigated by solid-state nuclear magnetic resonance, which indicates that crystalline stems and side-chain conformations are completely fixed up to melting points (〈τ_c〉 >10 s). In addition, lamellar thickness, 〈l〉 of iPB1 and a low isotacticity iPB1 (low_iPB1) with 〈mmmm〉=78%, respectively, were investigated by small-angle X-ray scattering. The low_iPB1 sample shows very week supercooling dependence of 〈l〉 (∼5 nm), whereas iPB1 shows strong supercooling dependence of 〈l〉 (10–28 nm). On the basis of molecular dynamics and 〈l〉 results, molecular dynamics effects on structures and unique mechanical property of iPB1 are discussed.