Triblock copolymer methoxy poly(ethylene glycol)-b-poly(dimethylaminoethyl methacrylate)-b-poly(-vinylpyridine) (MPEG-b-PDMAEMA-b-P4VP) was synthesized by atom transfer radical polymerization. It could self-assemble into micelles in aqueous solution. Superparamagnetic Fe3O4 particles were easily incorporated into the inner core due to the effect of complexation. The ionic bonding between the cationic part of PDMAEMA and anionic drug sodium fluorescein (SFS) led to a tight loading of SFS and pH-controlled release. The coloading ability of this copolymer indicates its potential application in theranostics.