The formation mechanism of non-banded polymer spherulites has been examined experimentally for isotactic poly(butene-1) grown from the melt by optical, atomic force, and transmission electron microscopies associated with quenching and chemical etching. At the growth front of the spherulites, the maximum width of lamellar crystals, lambda(m), showed a square-root dependence on the growth rate. The dependence suggests an instability-driven branching. In terms of the correlation of lamellar orientation in the spherulites, an auto-correlation function has been determined from the image taken by polarizing optical microscopy. The correlation showed an exponential decay along the radial direction, and the correlation length was in proportion to,,. Those experimental evidences suggest that the structure is formed by the coupling of the branching instability and the random re-orientation of lamellar crystals on the occasion of branching in the non-banded spherulites of poly(butene-1). (c) 2008 Elsevier Ltd. All rights reserved.