Tumor-targeting polymeric vesicles were prepared using a mixture of two amphiphilic block copolymers, namely, folate-poly(ethylene glycol)-poly(D,L-lactide) (folate-PEG-PDLLA) and NH₂-PEG-PDLLA. The copolymers were synthesized through sequential anionic polymerization in a well-controlled manner. Hydrophilic superparamagnetic iron oxide nanoparticles (SPIONs) were encapsulated in the aqueous core of the polymeric vesicles. Similar to the hydrophilic SPIONs, the SPION-loaded polymeric vesicles exhibited the superparamagnetic property. Moreover, the SPION-loaded polymeric vesicles demonstrated a higher r₂/r₁ value than a commercially available T₂ agent, Resovist^®, which can be attributed to the high SPION loading level and the SPION clustering effect in the aqueous core of the vesicles. Thus, tumor-targeting, SPION-loaded polymer vesicles can serve as highly efficient T₂ contrast probes for cancer diagnosis and treatment.