Novel homogenous Fe3O4 core-shell polyhedrons have been successfully fabricated through a simple cetyltrimethyl ammonium bromide (CTAB)-modified hydrothermal method. The formation of these Fe3O4 core-shell polyhedrons is based on a mechanism analogue to Ostwald ripening process, in which solid aggregates of Fe3O4 nanoparticles were first formed and then gradual outward migration of interior crystals during hydrothermal treatment divided the pristine aggregate into two discrete parts. Further recrystallization of the exterior part results in single-crystal polyhedral shell. Meanwhile, the core region would be further trimmed down to smaller size and detach from the shell. This novel structure with magnetic nature can be potentially applied in wide range of fields such as micro-reactor, delivery vehicle system and so on. (C) 2007 Elsevier B.V. All rights reserved.