Presented in this article is the preparation of a new theranostic vesicle which exhibits excellent in vitro and in vivo T-1 magnetic resonance (MR) imaging contrast effect and good anticancer drug delivery ability. The theranostic vesicle has been easily prepared based on an amphiphilic biocompatible and biodegradable dibock copolymer, poly(ethylene glycol)-block-poly(L-lactic-co-glycolic acid) (PEG-b-PLGA) and bovine serum albumin-gadolinium (BSA-Gd) complexes. Dynamic light scattering (DLS), transmission electron microscopy (TEM), UV-vis spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements confirmed the formation and physiological stability of BSA-Gd@PEG-b-PLGA vesicles. Furthermore, the in vitro and in vivo MR imaging experiments revealed their excellent T-1-weighted MR imaging function. Red blood cell hemolysis and cytotoxicity experiments confirmed their good blood compatibility and low cytotoxicity. Doxorubicin (DOX) loading and release experiments indicated a more retarded release rate of DOX in those theranostic vesicles than sole PEG-b-PLGA nanoparticles without BSA. Overall, this new biocompatible and biodegradable vesicle shows promising potential in theranostic applications.