Large-scale synthesis of monodisperse ultrasmall metal ferrite nanoparticles as well as understanding the correlations between chemical composition and MR signal enhancement is critical for developing next-generation, ultrasensitive T magnetic resonance imaging (MRI) nanoprobes. Herein, taking ultrasmall MnFeO nanoparticles (UMFNPs) as a model system, we report a general dynamic simultaneous thermal decomposition (DSTD) strategy for controllable synthesis of monodisperse ultrasmall metal ferrite nanoparticles with sizes smaller than 4 nm. The comparison study revealed that the DSTD using the iron-eruciate paired with a metal-oleate precursor enabled a nucleation-doping process, which is crucial for particle size and distribution control of ultrasmall metal ferrite nanoparticles. The principle of DSTD synthesis has been further confirmed by synthesizing NiFeO and CoFeO nanoparticles with well-controlled sizes of ∼3 nm. More significantly, the success in DSTD synthesis allows us to tune both MR and biochemical properties of magnetic iron oxide nanoprobes by adjusting their chemical composition. Beneficial from the Mn dopant, the synthesized UMFNPs exhibited the highest r relaxivity (up to 8.43 mM s) among the ferrite nanoparticles with similar sizes reported so far and demonstrated a multifunctional T MR nanoprobe for in vivo high-resolution blood pool and liver-specific MRI simultaneously. Our study provides a general strategy to synthesize ultrasmall multicomponent magnetic nanoparticles, which offers possibilities for the chemical design of a highly sensitive ultrasmall magnetic nanoparticle based T MRI probe for various clinical diagnosis applications.