Triblock copolymers of monomethoxy poly(ethylene glycol) (mPEG) and ε-caprolactone (CL) were prepared with varying lengths of poly(ε-caprolactone) (PCL) compositions and a fixed length of mPEG segment. The molecular characteristics of triblock copolymers were characterized by 1H NMR, gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). These amphiphilic linear copolymers based on PCL hydrophobic chain and hydrophilic mPEG ending, which can self-assemble into nanoscopic micelles with their hydrophobic cores, encapsulated doxorubicin (DOX) in an aqueous solution. The particle size of prepared micelles was around 40-92 nm. The DOX loading content and DOX loading efficiency were from 3.7-7.4% to 26-49%, respectively. DOX-released profile was pH-dependent and faster at pH 5.4 than pH 7.4. Additionally, the cytotoxicity of DOX-loaded micelles was found to be similar with free DOX in drugresistant cells (MCF-7/adr). The great amounts of DOX and fast uptake accumulated into the MCF-7/adr cells from DOX-loaded micelles suggest a potential application in cancer chemotherapy.