Doping　Mn　(II)　ions　into　iron　oxide　(IO)　as　manganese　ferrite　(MnIO)　has　been　proved　to　be　an　effective　strategy　to　improve　T1　relaxivity　of　IO　nanoparticle　in　recent　years;　however,　the　high　T2　relaxivity　of　MnIO　nanoparticle　hampers　its　T1　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　T1　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　T1　relaxivity　of　9.1　mM−1s−1　at　3　T　magnetic　field.　Moreover,　exclusive　engineering　the　interfacial　structure　significantly　lower　the　T2　enhancing　effect　caused　by　doped　Mn　(II)　ions,　which　further　limits　the　impairing　of　increased　T2　relaxivity　to　T1　contrast　imaging.　IO@MnIO　nanoparticles　with　different　shell　thicknesses　reveal　comparable　T1　relaxation　rates　but　obvious　lower　T2　relaxivities　and　r2/r1　ratios　to　MnIO　nanoparticles　with　similar　sizes.　The　desirable　T1　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　T1　contrast　and　a　new　guidance　for　designing　effective　high-performance　T1　contrast　agent　for　early　cancer　diagnosis.　©　2022　Elsevier　Inc.