Monodisperse iron oxide nanoparticles were prepared by thermal decomposition of iron carbonyl in octyl ether in the presence of oleic acid. The particle size could be tuned from 7 nm to 25 nm. The dual role of oleic acid as a surfactant and a boiling-point elevating agent was discussed. The impacts of varied reflux conditions were investigated and a new monodisperse mechanism was given. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were used to study the oxidation states of iron oxide nanoparticles. The evolution of d-spacing values in HRTEM images and SAED patterns demonstrated the transition from γ-FeO to FeO with the increasing particle size. Superconducting quantum interference device (SQUID) and vibrating sample magnetometer (VSM) were used to reveal the superparamagnetic behavior of as-synthesized nanoparticles. © 2011 Elsevier B.V. All rights reserved.