We report a facile synthesis of water-soluble, ultrasmall, PEGylated MnxFe3-xO4 nanoparticles (MFNPs) (x = 0-0.34) and the Mn(ii) doping effects on T1- and T2-weighted magnetic resonance imaging (MRI). By adjusting the reaction conditions, the 'x' value can be continuously tuned from 0 to 0.34. The produced MFNPs are of high crystallinity and size uniformity with an average diameter of ∼6 nm, which show excellent colloidal stability in H2O, PBS, and tolerate a high salt concentration (1 M NaCl) and a wide pH range from 7 to 11. The results of FTIR demonstrate that both HOOC-PEG-COOH and TEG were modified on the nanocrystal surfaces. The saturation magnetization of the MFNPs gradually increases with increasing Mn2+ concentration and reaches 75.5 emu g-1 for x = 0.34. Careful investigation of the Mn(ii) doping effects on T1- and T2-weighted MRI reveals that T2 contrast effects are enhanced while T1 contrast effects are weakened with the increase of the 'x' value of the MFNPs. Furthermore, the T1 contrast effects of the MFNPs are concentration dependant. A concentration which is lower than 0.500 mM is needed for the MFNPs to act as T1 and T2 dual contrast agents at 3 T.