Abstract: AIM To investigate the possibility of poly(lactic-co-glycolic acid) as a carrier for the delivery of macromolecular. METHODS Insulin-loaded poly(lactic-co-glycolic acid) nanoparticles (INS-PLGA-NPs) was prepared by a double-emulsion solvent evaporation method. The size distribution was examined by photo-correlation spectrometry. The entrapment efficiency was determined by HPLC and important factors that affected the entrapment efficiency were investigated. The loading mechanism of different size nanoparticles was assayed by radioimmunoassay (RIA). INS-PLGA-NPs release behavior in vitro was carried out under sink condition. After oral administration of the nanoparticles to alloxan-induced diabetic rats, its glucose level was determined by glucose oxidize method and the oral pharmacological bioavailability in contrast to sc of insulin solution was calculated according to the area over the curve. RESULTS The INS-PLGA-NPs was prepared with poloxamer 188 as a emulsifier, the mean diameter was 149.6 nm and the polydispersity index was decreased to 0.09. While the entrapment efficiency was increased to 42.8%. Most of the insulin loaded was adsorbed on the surface of the nanoparticles. The release behavior in vitro showed an initial burst effect followed by a slower rate stage. After oral administration of 10 u·kg^-1 INS-PLGA-NPs, the plasma glucose level decreased significantly after 4 h (P<0.05), 10 h later the glucose level decreased to the lowest (52.4%±10.2%, P<0.01) and the relative pharmacological bioavailability is (10.3±0.8)%. CONCLUSION PLGA-NPs might be used as a new oral carrier for protein drug delivery systems in the future.