In present work, we synthesized monodisperse Fe3O4 microsphere using a free-surfactant solvothermal reduction route and investigated effect of the precursor concentration (FeCl3∙6H2O and NaAc) on microsphere sizes under other reaction conditions held constant. The morphologies, structures, and magnetism of the products were characterized by SEM, XRD, FTIR and VSM. The results showed that the Fe3O4 microsphere with a tunable average size range from 120 nm to 300 nm is composed of many Fe3O4 collective nanoparticles. Their average diameters increased with increase of precursor FeCl3∙6H2O concentration but decreased with increase of precursor NaAc concentration. The X-ray diffraction (XRD) pattern confirmed that the Fe3O4 microspheres belong to cubic structure. Magnetic investigation reveals that the Fe3O4 microspheres have higher saturation magnetization and negligible coercivity at room temperature.