In this work, an amphiphilic diblock copolymer (PEG(43)-b-PSDTE(29)) bearing photochromic dithienylethene (DTE) pendants is synthesized by reversible addition fragmentation chain transfer radical polymerization. The diblock copolymer was characterized by spectroscopic methods and gel permeation chromatography. The analyses proved the well-defined structure and narrow molecular weight distribution of the diblock copolymer. The DTE pendants could undergo reversible photoisomerization between their open and closed forms in solution when irradiated with UV and visible light as indicated by (1)H NMR and UV-vis spectroscopy. Hollow vesicle-like structures were formed by gradually adding deionized water to the colorless PEG(43)-b-PSDTE(29open) (DTE in open form) tetrahydrofuran solution. Under the same conditions, the aggregates formed in the blue PEG(43)-b-PSDTE(29close) (DTE in closed form) solution were colloidal spheres with solid interiors. The isomerization of DTE pendants could cause the deformation of the vesicle-like structures. The above results demonstrate a kind of novel photo-modulated self-assembly behavior of the amphiphilic diblock copolymer, which could be used for drug-delivery and other applications. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.