RNA interference is an effective method to achieve highly specific gene regulation. However, the commonly used cationic liposomes have poor biocompatibility, which may lead to systematic siRNA delivery of no avail. PEGylation is a good strategy in shielding the positive charge of cationic liposomes, but the enhanced serum stability is often in company with compromised cellular uptake and endosome escape. In this study, PEG was covalently linked to negatively charged hyaluronic acid and it was used to coat the liposome-siRNA nanoparticles. The resulting PEG-HA-NP complex had a diameter of 188.6 ± 10.8 nm and a dramatically declined zeta-potential from +34.9 ± 4.0 mV to -18.2 ± 2.2 mV. Owing to the reversed surface charge, PEG-HA-NP could remain stable in fetal bovine serum (FBS) to up to 24h. In contrast with normal PEGylation, hyaluronic acid and PEG co-modified PEG-HA-NP provided comparable cellular uptake and P-glycoprotein downregulation efficacy in MCF-7/ADR cells compared with Lipofectamine RNAiMAX and naked NP regardless of its anionic charged surface. Because of its good biocompatibility in serum, PEG-HA-NP possessed the best tumor accumulation, cellular uptake and subsequently the strongest P-glycoprotein silencing capability in tumor bearing mice compared with naked NP and HA-NP after i.v. injection, with a 34% P-glycoprotein downregulation. Therefore, PEG-HA coated liposomal complex was demonstrated to be a promising siRNA delivery system in adjusting solid tumor P-glycoprotein expression, which may become a potential carrier in reversing MDR for breast cancer therapy.