MnFe2O4 nanoparticles are synthesized by using sodium dodecylbenzenesulfonate (NaDBS) to form water-in-toluene reverse micelles. The nanoparticles are single crystalline, and the average particle size can be controlled from 4 to 15 nm. High- and low-resolution transmission electron microscopy characterization has shown that the nanoparticles can have a size distribution as narrow as similar to 9%. Neutron diffraction and magnetic measurements have been conducted on the nanoparticles with a diameter of 7.7 +/- 0.7 nm. The results unambiguously prove that these MnFe2O4 nanoparticles are truly superparamagnetic. The synthesis and characterization of these nanoparticles will facilitate the development of MnFe2O4 nanoparticles for the potential applications such as contrast enhancement agents of magnetic resonance imaging and magnetic carriers for site-specific drug delivery.