Exploration of a green carrier to avoid potential systemic toxicity and the unclear metabolic mechanism of traditional nanocarriers is of high importance for cancer therapy. Hence, we developed a carrier-free nanosystem for co-delivery of dual anti-cancer drugs ursolic acid (UA) and doxorubicin (DOX) using a "green" and simple method. The co-assembled nanodrug was further modified with a HER2 aptamer by electrostatic interactions. The co-assembled dual nanodrug presented a spherical morphology with a uniform size (∼108.9 nm) and in a pH-triggered drug release manner. It made UA sensitize DOX to display synergistic anticancer effects at a low dose of DOX. Further, the aptamer surface decoration improved the intracellular drug retention of UA and DOX to as much as 2-fold in HER2 overexpressing cancer cells. In addition, the in vivo results further proved that the co-assembled nanodrug could significantly inhibit the tumor growth with a little side effects. In a word, this novel carrier-free dual-drug nanodelivery system could be a potential drug candidate for HER2 overexpressing cancer therapy, and UA could be used as a "green" nanocarrier for delivery of hydrophobic drugs and fluorescent dyes in cancer treatment and diagnosis.