Abstract: OBJECTIVE To develop paclitaxel(PTX)/MPEG-PCL nanoparticles and investigate its in vitro release behavior. METHODS MPEG-PCL block polymer was synthesized by a ring-opeing polymerization method, followed by the characterization with 1H-NMR and FTIR. The PTX/MPEG-PCL nanoparticles was firstly prepared by a coprecipitation method, and then characterized by a DLS, TEM and etc. In vitro release study of PTX/MPEG-PCL nanoparticles as a function with time was performed in PBS solution (pH=7.4) at 37 ℃. RESULTS With the analysis of 1H-NMR and FTIR spectrum, MPEG-PCL block polymer (molecular weight=4 875) was successfully synthesized. With the observation of TEM, the developed PTX/MPEG-PCL nanoparticles showed almost spherical in shape with uniform mean particle size about (102.3±3.5)nm. Drug loading effciency and drug loading capacity of PTX/MPEG-PCL nanoparticles were (95.6±3.2)% and (8.5±0.4)%, respectively. In vitro release study indicated that PTX was released in a sustained-release manner from MPEG-PCL nanoparticles. CONCLUSION MPEG-PCL nanoparticle as an excellent carrier for PTX exhibits high drug loading effciency, drug loading capacity and sustained release progerty.