Monodisperse Fe3O4 microspheres assembled by a number of nanosize tetrahedron subunits have been selectively synthesized through the hydrothermal process. The synthesized Fe3O4 microspheres have good dispersibility. The subunits made up of microspheres were uniform in size and like-tetrahedron in shape. The average diameter of each Fe3O4 microsphere is about 50-55 mu m. The length of each edge of tetrahedron is about 100 nm. A series of experiments had been carried out to investigate the effect of reductant, precipitator and reaction time on the formation of Fe3O4 microsphere and tetrahedron subunits. The results show that ascorbic acid as reductant and urea as precipitator supplied a relatively steady environment during the synthesis process and led to the formations of Fe3O4 tetrahedron subunit and monodisperse Fe3O4 microspheres. As the reaction time increased from 3 to 24 h, the Fe3O4 microspheres tended towards dispersion and becoming large in size from 10-20 to 50-55 mu m, and the subunits formed Fe3O4 microspheres that varied from spheroid to tetrahedron and from a small nanoparticle (20-30 nm) to a large one (90-110 nm). A reasonable explanation for the formations of the Fe3O4 microsphere and the tetrahedron subunit was proposed through Ostwald ripening and the attachment growth mechanism, respectively. (C) 2009 Elsevier B.V. All rights reserved.