Iron oxide nanoparticles (IONPs) have been developed as contrast agents for - or -weighted magnetic resonance imaging (MRI) on account of their excellent physicochemical and biological properties. However, general strategies to improve longitudinal relaxivity () often decrease transverse relaxivity (), thus synchronously strengthening the and enhancement effect of IONPs remains a challenge. Here, we report interface regulation and size tailoring of a group of FePt@FeO core-shell nanoparticles (NPs), which possess high and relaxivities. The increase of and is due to the enhancement of the saturation magnetization (), which is a result of the strengthened exchange coupling across the core-shell interface. In vivo subcutaneous tumor study and brain glioma imaging revealed that FePt@FeO NPs can serve as a favorable - dual-modal contrast agent. We envision that the core-shell NPs, through interface engineering, have great potential in preclinical and clinical MRI applications.