The crystalline morphology in thin and ultrathin films of linear low-density polyethylene is reported here. Films with thickness ranging from 20 to 800 nm were isothermally crystallized at temperatures ranging from 90 to 119 degrees C, and the supermolecular and lamellar morphologies were observed by contact and tapping mode atomic force microscopies. Four distinct supermolecular morphologies were observed: banded spherulite, sheaflike, and two intermediate morphologies. The morphological transitions were significant in the films thinner than 200 nm. The sheaflike morphology occurred in the thinner films (<200 nm) while the banded spherulite morphology occurred in the thicker films. The change of morphology was primarily attributed to the change in the extent of lamellar branching. With increased branching, more edge-on lamellae were observed and resulted in changes in the morphology of the thin films. The morphological transitions are discussed on the basis of the chain structure and the unique physical properties of the thin and ultrathin films.