Traditional T-1 magnetic resonance imaging (MRI) contrast agents such as diethylenetriaminepentacetatic acid (DTPA) chelated gadolinium [Gd(III)] have poor sensitivity, leading to a risk of accumulated toxicity in vivo. To significantly improve the sensitivity of a T-1 MRI contrast agent and to enhance the efficacy of cancer chemotherapy, herein for the first time we report a noncytotoxic asymmetrical cancer targeting polymer vesicle based on R-poly(L-glutamic acid)-block-poly(epsilon-caprolactone) [R is folic acid (FA) or DTPA]. Such asymmetrical vesicles have a cancer-targeting outer corona and a Gd(III)-chelating and drug-loading-enhancing inner corona, exhibiting an extremely high T-1 relaxivity (42.39 mM(-1) s(-1), 8-fold better than DTPA-Gd) and anticancer drug loading efficiency (52.6% for doxorubicin hydrochloride, DOX center dot HCl). Moreover, the DOX-loaded vesicles exhibited excellent antitumor activity (2-fold better than free DOX). This chelating-just-inside strategy for synthesizing asymmetrical polymer vesicles demonstrated promising potential theranostic applications in magnetic resonance imaging and cancer-targeted drug delivery.