Manganese (Mn)-based nanoparticles have been proved to be promising MR T1 contrast agents for the diagnosis of brain tumors. However, most of them exhibit a low relaxation rate, resulting in an insufficient enhancement effect on tiny gliomas. Herein, we developed gadolinium (Gd)-doped MnCO3 nanoparticles with a size of 11 nm via the thermal decomposition of Mn-oleate in the presence of Gd-oleate. Owing to the small size and Gd doping, these Gd-doped MnCO3 NPs, when endowed with excellent aqueous dispersibility and colloidal stability, exhibited a high r1 relaxivity of 6.81 mM(-1) s(-1). Moreover, the Gd/MnCO3 NPs were used as a reliable platform to construct a glioma-targeted MR/fluorescence bimodal nanoprobe. The high relaxivity, the bimodal imaging capability, and the specificity nominate the multifunctional Gd doped MnCO3 NPs as an effective nanoprobe for the diagnostic imaging of tiny brain gliomas with an improved efficacy.