Abstract: Objective To prepare an insulin-loaded nanoparticle assembled using pH-sensitive poly(ethylene glycol)-poly (ε-caprolactone)-poly(N,N-diethylamino-2-ethylmethaerylate) (mPEG-PCL-PDEAEMA) and investigate its performance of sustained insulin release in vitro and its hypoglycemic effects in diabetic rats. Methods mPEG-PCL-PDEAEMA triblock copolymers with different hydrophobic lengths were synthesized by ring opening polymerization (ROP) combined with atom transfer radical polymerization (ATRP). The copolymers were characterized using Fourier-transform Infrared (FT-IR) spectroscopy and proton nuclear magnetic resonance spectroscopy (1H-NMR). Insulin-loaded nanoparticles were prepared by nanoprecipitation technique, in which the reversible swelling of the pH-sensitive material was used for insulin loading and release. The obtained nanoparticles were further confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The entrapment efficiency (EE%), drug loading (DL%) and in vitro release characteristics of the insulinloaded nanoparticles were assessed using BCA protein assay kit. The hypoglycemic effects of the nanoparticles were evaluated by monitoring the glucose levels. Results The size of the nanoparticles decreased as pH value increased within the range of 1.2 to 7.4. Using copolymers mPEG5k-PCL13k- PDEAEMA10k and mPEG5k-PCL10k-PDEAEMA10k as the drug carriers, the nanoparticles prepared with an optimal insulin-coplymer mass ratio of 90% had an average size of 181.9±6.67 nm and 169±7.1 nm, maximal EE% of (81.99±1.77)% and (53.12±0.62)%, and maximal DL% of (42.46±0.53)% and (32.34±0.26)%, respectively. Compared with free insulin, the insulin-loaded nanoparticles was capable of sustained insulin release and the release rate was lowered as the hydrophobic length increases. In diabetic rats, the insulin-loaded nanoparticles based on mPEG5k-PCL13k- PDEAEMA10k maintained a sustained hypoglycemic effect for 48 h, which was significantly longer than the time of free insulin. Conclusion The pH-sensitive triblock copolymer mPEG-PCL-PDEAEMA can serve as a promising candidate of carrier for sustained release of insulin.