Core cross-linked (CCL) polymeric micelles covalently labeled with DOTA(Gd) and green- emitting NBD fluorophores within pH-responsive cores are fabricated, which can serve as a dual-modality MR/fluorescence imaging system and exhibit mildly acidic pH-triggered turn-on or enhancement of signal intensities for both imaging modalities. Compared to non-crosslinked diblock precursor, CCL micelles demonstrate better MR and fluorescence imaging performance due to structural stability and integrity endowed by the core cross-linking procedure. Furthermore, selective enhancement of MR/fluorescence imaging signal intensities of the dual-modality imaging system can also be actuated under specific intracellular microenvironments such as mildly acidic organelles.