Uniform Fe3O4 nanospheres have been synthesized by a simple ethylene glycol-H2O system without adding any additives. The average diameter of the Fe3O4 nanospheres was 120-190 nm, and the nanospheres are formed by nanoparticles of several tens of nanometers. The presence of a certain amount of deionized water and the controlled reaction temperature plays an important role in the formation of the Fe3O4 nanospheres. The formation mechanism of the nanospheres is discussed on the basis of the experimental results.