Folate-functionalized copolymers of poly(ethylene glycol) and 2-(diisopropylamino) ethylamine grafted poly(L-aspartic acid) are synthesized. The copolymers can self-assemble into nanoscaled micelles encapsulated with hydrophobic model drug Fluorescein Diacetate (FDA) and MRI diagnostic agents superparamagnetic iron oxide nanoparticles (SPIONs) in aqueous solution of a neutral pH resembling physiological environment, whereas disassemble in acidic endosomal/lysosomal compartments of tumor cells to achieve rapid drug release. In vitro drug release study showed that FDA release from the pH-sensitive micelles was much faster at pH 5.0 than at pH 7.4. Clustering of SPIONs inside the hydrophobic core of the micelles resulted in a high spin-spin (T2,) relaxivity for a super MRI sensitivity. Cell culture studies showed that the FDA-SPION-loaded micelles were effectively internalized by human hepatic Bel-7402 cancer cells following a folate receptor-mediated targeting mechanism, and then FDA was rapidly release from micelles inside lysosomal compartments. Micelles encapsulating paclitaxel (PTX) studies showed it can induce more effective cell toxicity. This study demonstrated the great potential of the pH-sensitive micelles as an effective multifunctional nanomedician platform for cancer therapy due to their active tumor targeting, pH-triggered drug release and ultrasensitive MRI responsiveness.