We report a nanoscale morphology-regulation strategy of self-assembled double-hydrophilic block copolymers with square planar PtII compounds. The selective coordination of PtII on the chelating blocks of poly(acrylic acid)-b-poly(ethylene glycol) (PAA-b-PEG) induced the self-association of metal-chelated unimers by the known cohesive force of PtII. The block-length variation of PAA with constant PEG led to the shape transition from normal core/shell and crew-cut spheres to anisotropic pearl-string structures. On the other hand, PtII adsorption on PEG blocks by extensive hydrogen bonding can further modify the molecular geometry of metal-chelated unimers by decreasing the volume of hydrophilic segments, eventually leading to the shape transition to vesicular structures. This result was well correlated to the structural constraint of PEG conformation estimated by the quantitative 1H NMR analysis. The vesicles also exhibited the enclosing nature for the fluorescent guest molecules, which demonstrated the promising potential for the encapsulating delivery vehicle.