Uniform and single-crystalline Mn-Co-Ni-O nanocrystallines (MCN NCs) are successfully synthesized by the reverse microemulsion (RM) process. The microstructure, morphology and particle size of MCN NCs are characterized by Fourier Transform Infrared (FT-IR), X-ray powders diffraction (XRD), scanning electron microscope (SEM) and (High Resolution) Transmission Electron Microscope (HRTEM), respectively. These results show that the mole ratio of water to surfactant and initial reagents concentration play essential roles in the formation of cubic and tetragonal spinel MCN NCs with different morphologies of nano-sphere, nano-octahedra, and nano-hexagonal prism. Finally, the changing rules of microstructure and morphology are clarified, which help us understand the formation mechanism of spinel oxides. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.