Superparamagnetic Fe3O4 hollow microspheres with an average diameter of 240 nm have been solvothermally synthesized where each of them is composed of primary nanoparticles with the size of 10-14 nm. Based on electron microscopy technique, the morphological evolution with the reaction time is observed. Since the transformation from solid to hollow spheres was perceived, Ostwald ripening process is proposed to explain the formation of hollow microspheres. In particular, the magnetism investigation indicates that the as-prepared samples remain room-temperature superparamagnetic behavior with a high saturation magnetization (up to 83.5 emu/g), which results from the separation of the primary magnetic nanoparticles. The excellent products may lead to many promising applications in enzyme immobilization, cancer therapies, and MRI contrast agents, and environment-friendly. (C) 2011 Elsevier B.V. All rights reserved.