The - dual-mode contrast agents for magnetic resonance imaging (MRI) can generate self-complementary confirmed and images, hence greatly improving the reliability. Facilely synthesizing nanoparticles with the ultrasensitive contrast property remains extremely challenging in nanoscience. Moreover, uncovering the mechanism correlating the signal enhancements and chemical constituents is vital for designing novel efficient synergistically enhanced - dual-mode MRI nanoprobes. Herein, we report a one-pot facile method to synthesize the superparamagnetic manganese oxide-doped iron oxide (FeO/MnO) nanoparticles for - dual-mode MR imaging. Under external magnetic field, the local magnetic field intensities of MnO and FeO could be simultaneously enhanced through embedding MnO into FeO nanoparticles and hence can cause synergistic and contrast enhancements. Moreover, a novel and facile cost-effective method for large-scale synthesis of hydroxyl-polyethylene glycol-phosphonic acid-stabilizing ligands is designed. The facile synthetic method and surface coating strategy of superparamagnetic FeO/MnO nanoparticles offer an idea for the chemical design and preparation of superparamagnetic nanoparticles with ultrasensitive MRI contrast abilities for disease evaluation and treatment.