We are herein reporting a synthesis of indomethacin-loaded bilayer-surface magnetite nanoparticles and their releasing behavior. The particles were first stabilized with oleic acid as a primary surfactant, followed by poly(ethylene glycol) methyl ether-poly(epsilon-caprolactone) (mPEG-PCL) amphiphilic block copolymer as a secondary surfactant to form nanoparticles with hydrophobic inner shell and hydrophilic corona. mPEG-PCL copolymers with systematically varied molecular weights of each block (2000-2000, 2000-10,000, 5000-5000 and 5000-10,000g/mol, respectively) were synthesized via a ring-opening polymerization of epsilon-caprolactone using mPEG as a macroinitiator. The particles were 9 nm in diameter and exhibited superparamagnetic behavior at room temperature with saturation magnetization (M-s) about 35 emu/g magnetite. Percent of magnetite and the copolymers in the complexes were determined via thermogravimetric analysis (TGA). The effect of mPEG and PCL block lengths in the copolymer-magnetite complex on the properties of the particles, e.g. particle size, magnetic properties, stability in water. drug entrapping and loading efficiency and its releasing behavior were investigated. This novel magnetic nanocomplex might be suitable for use as an efficient drug delivery vehicle with tunable drug-released properties. (C) 2009 Elsevier Ltd. All rights reserved.