In order to develop a prolonged circulating drug carrier and to overcome p-glycoprotein-mediated multidrug resistance to adriamycin (ADR), which is a potent chemotherapeutic agent in the treatment of various cancers, poly(lactide-co-glycolide)-polyethylene glycol (PLGA-PEG) nanoparticles were prepared and characterized. ADR-loaded PLGA-PEG nanoparticles prepared by the emulsification-diffusion method were spherical and homogeneous with smooth surfaces when assessed by scanning electron microscopy. The nanoparticles were 200-230 nm in size and the encapsulation efficiency of ADR in the nanoparticles was 30 approximately 35%. The release of ADR from nanoparticles was extended compared to that from free ADR solution. After intravenous administration of adriamycin-loaded PLGA-PEG nanoparticles to rats, the plasma level of ADR from PLGA-PEG nanoparticle was extended until 24 hours and the mean residence time of ADR of nanoparticles was increased compared to that of ADR solution. And ADR-loaded nanoparticles showed a higher growth inhibitory effect than free ADR solution in an ADR resistant MCF-7 human breast carcinoma cell line. The prepared ADR-loaded PLGA-PEG nanoparticles can be used as a good delivery system for ADR.