Oleanolic acid (OA) has shown promising antitumor activity. However, it is difficult to formulate because of its poor water solubility and low bioavailability. In this study, self-assembled nanoparticles based on poly(ethylene glycol)–oleanolic acid conjugates (mPEG–OA) for co-delivery of anticancer drugs were developed to enhance the properties of OA. The mPEG–OA was synthesized by conjugating oleanolic acid (OA) to methoxy poly(ethylene glycol) carboxylic acid (mPEG–COOH). Then hydroxycamptothecin (HCPT), another anticancer drug was encapsulated into the mPEG–OA conjugates by the nanoprecipitation method. The mPEG–OA/HCPT nanoparticles (NPs) had an appropriate size, high drug loading of OA and HCPT, high stability, slow drug release and low cytotoxicity. In vivo studies revealed that the nanoparticles significantly improved the antitumor activity compared with free drugs. As no systemic toxicity was observed, the mPEG–OA NPs can serve as a promising drug delivery system for cancer therapy.