We demonstrate a tumor mRNA-dependent drug carrier for controlled release of doxorubicin (Dox) and intracellular imaging based on gold nanoparticle–molecular beacon. Fluorescent Dox is released effectively and induces apoptosis in breast cancer cells but not in normal cells. Significantly, the release of Dox is correlated positively with the quantities of tumor mRNA, which is according to various stages of tumor progression, and so can decrease effectively side effects of Dox.