Monodisperse Fe3O4 nanoparticles (NPs) with a narrow size distribution were synthesized by a high-temperature solution-phase method. The diameter of magnetic NPs was tuned from 2 to 13 nm by varying the reaction conditions, and the monolayer and multilayer self-assemblies of the NPs were obtained. A few kinds of the assemblies are presented and the reasons that drive the particles to form the superlattices are discussed. The physical-property-measurement system (PPMS) results showed that the as-synthesized NPs are ferrimagnetic at low temperatures (5 K) while they are superparamagnetic at room temperature (300 K). In addition, the particles were transformed into hydrophilic ones by an exchange reaction. Copyright © 2006 John Wiley & Sons, Ltd.