Monodispersed ferrite nanospheres were synthesized in an ethylene glycol/diethylene glycol (EG/DEG) binary solvent by using poly(vinylpyrrolidone) (PVP) as the surfactant. Particle size control can be attained by careful adjustment of the VEG/VDEG ratio. Different from previous reports, the magnetic studies of our iron ferrite (Fe3O4) nanospheres with various sizes from 20 to 300 nm reveal that they exhibit a similar magnetic saturation (Ms) value. In particular, both the ferromagnetic and superparamagnetic Fe3O4 nanospheres with similar Ms can be selectively obtained by varying the H2O concentration in the reaction system. The magnetic resonance imaging (MRI) characterization indicates that the as-prepared superparamagnetic Fe3O4 and ZnFe2O4 nanospheres possess a T2 relaxivity range that can be used as potential MRI contrast agents.