Traditional chemotherapeutic drugs have shown limited clinical curative effects in antitumor therapy. The application of multidrug combination and adjuvant-drug carriers is a feasible strategy to overcome the limitations while minimizing the dosage of single drug and acquiring the synergistic effects in tumor therapy. However, the systemic toxicity, drug resistance, and tumor recurrence are still unavoidable. Here we develop core–shell nanoparticles (NPs) to encapsulate paclitaxel (PTX) and gemcitabine (GEM) for breast cancer therapy. We find that the NPs could encapsulate PTX and GEM, with an encapsulation efficiency of 96.3 and 95.13%, respectively. Moreover, the drug loading of these NPs is 2.71% (PTX) and 2.64% (GEM). Notably, the co-delivery of GEM and PTX performs enhanced anticancer effect compared with the PTX alone or GEM alone therapy at the same concentration, which indicates a synergistic effect. Moreover, encapsulation of PTX and GEM by methoxy poly(ethylene glycol)–poly(lactide-coglycolide) also shows enhanced anticancer effects (81.5% tumor inhibition) and reduced systemic toxicity in vivo compared with free drugs (65% tumor inhibition). Together with those results, co-delivery of PTX and GEM by methoxy poly(ethylene glycol)–poly(lactide-coglycolide) might have important potencies in clinical applications for breast cancer therapy.