We present a systematic study on the preparation, microstructure, and magnetic properties of Fe3O4 microspheres and Fe3O4@SiO2 microspheres. Results showed that Fe3O4 microspheres' diameter can be tuned by Fe3+ concentration, whereas their average grain size can be tuned by polyethylene glycol (PEG) 2000 dosage or PEG molecular weight. The magnetic saturation value of Fe3O4 microspheres was observed to be dependent on their average grain size, but not the sphere diameter. Fe3O4@SiO2 microspheres with different magnetic saturation values were achieved by adjusting shell thickness. Furthermore, the synthesized Fe3O4 and Fe3O4@SiO2 microspheres with high and controllable magnetic saturation value endow them with great application potentials.