The morphology of a H-shaped block copolymer (poly(ethylene glycol) backbone and polystyrene branches (PS)(2)PEG(PS)(2)) in a thin film has been investigated. A peculiar square lamella that has a phase-separated microdomain at its surface is obtained after spin coating. The experimental temperature plays a critical role in the lamellar formation. The copolymer first self-assembles into square lamellar micelles with an incomplete crystalline core due to the crystallizability of PEG. In the subsequent process of solvent evaporation, phase separation between PS and non-crystalline PEG attached at the chain-folded PEG surface takes place, which results in phase-separated microdomains being formed at the lamellar surface.