In this paper, the crystallization of poly(2-vinyl pyridine)-block-poly(ethylene oxide) (P2VP-b-PEO) block copolymer micelles with an amorphous P2VP core and crystallizable PEO corona upon cooling in n-amyl acetate/n-butanol (NAA/BOL) mixed solvent was investigated. The micellar solutions of the block copolymers were quenched from 65 to -33 A degrees C first and then to the crystallization temperature -3 A degrees C. Transmission electron microscopy was used to observe the morphological changes of the block copolymers. The influence of the solubility or chain mobility of the P2VP core on the formation of square crystals was investigated. Square crystals of uniform size and shape in large quantities could be obtained. The crystals are composed of PEO single-crystal layer sandwiched between two P2VP layers tethered on the top and bottom basal surfaces. The crystallization mechanism that spherical micelles transform into squared crystals is discussed.