Doping Mn (II) ions into iron oxide (IO) as manganese ferrite (MnIO) has been proved to be an effective strategy to improve T relaxivity of IO nanoparticle in recent years; however, the high T relaxivity of MnIO nanoparticle hampers its T contrast efficiency and remains a hurdle when developing contrast agent for early and accurate diagnosis. Herein, we engineered the interfacial structure of IO nanoparticle coated with manganese ferrite shell (IO@MnIO) with tunable thicknesses. The Mn-doped shell significantly improve the T contrast of IO nanoparticle, especially with the thickness of ∼0.8 nm. Compared to pristine IO nanoparticle, IO@MnIO nanoparticle with thickness of ∼0.8 nm exhibits nearly 2 times higher T relaxivity of 9.1 mMs at 3 T magnetic field. Moreover, exclusive engineering the interfacial structure significantly lower the T enhancing effect caused by doped Mn (II) ions, which further limits the impairing of increased T relaxivity to T contrast imaging. IO@MnIO nanoparticles with different shell thicknesses reveal comparable T relaxation rates but obvious lower T relaxivities and r/r ratios to MnIO nanoparticles with similar sizes. The desirable T contrast endows IO@MnIO nanoparticle to provide sufficient signal difference between normal and tumor tissue in vivo. This work provides a detailed instance of interfacial engineering to improve IO-based T contrast and a new guidance for designing effective high-performance T contrast agent for early cancer diagnosis.