Fe3O4 nanoparticles and alpha-FeOOH nanorods with tunable size have been selected control synthesized via a facile hydrothermal process in the presence of CTAB. By means of adjusting the volume ratio of ethanol/water media at three levels: 0 (in which water was used as a solvent alone), 2:3, and 4:1, phases of the products transformed from Fe3O4 into alpha-FeOOH gradually. X-ray powder diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy were used to characterize the as-synthesized samples. The related mechanisms of phase transformation are discussed.