Monodisperse magnetite nanospheres with hollow interior structure were synthesized through one-pot solvothermal process, in an isothermal environment at 200 degrees C for 12 h, using a sole iron precursor (FeCl3.6H(2)O) and without any template. We demonstrated the development of hollow structure of magnetite spheres by characterizing systematically the changes of morphology and crystal structure for different processing times. We also provided the cross-sectional images of the Fe3O4 spheres at different processing times to visualize the hollowing process inside the spheres with time. A detailed process mechanism to form the hollow structure of magnetite spheres was proposed, combining the formation of numerous tiny grains, the spherical assembly of those grains and the chemical conversion of the Fe (III) compounds to generate Fe3O4 simultaneously coupled with the Ostwald ripening process within the magnetite spheres. (c) 2013 American Institute of Chemical Engineers AIChE J, 59: 3594-3600, 2013