Nongadoliniumbased T₁  contrast agents have attracted great interest in recent years. In this work, (3.02±0.62) nm zincdoped iron oxide nanoparticles (ZnFe₂O₄) with a high r₁ relaxivity of 2.5 L/（mmol·s） and low r₂/r₁ ratio of 2.2 are synthesized using a onepot thermal decomposition process. The synthetic procedure is simple, cost effective, and easy to scale up. The nanoparticles are modified with a biologically compatible ligand POPEG. Due to the hydrophilic polymer coating, the particles are highly dispersible and stable in aqueous solution. Finally, we applied the nanoparticles to in vivo magnetic resonance imaging and obtained a relative signal enhancement of 11% at the tumor region. This study revealed ZnFe₂O₄^@PO-PEG as a potential candidate for a nongadoliniumbased T₁ contrast agent with extraordinary biosafety.