Mn-Co-Ni-O ultrafine ceramic nanoparticles are successfully synthesized by the reverse microemulsion method in a triton X-100/n-hexanol/cyclohexane system. The thermal decomposition process, crystallinity and crystal structure of Mn-Co-Ni-O powders are investigated and characterized by thermogravimetric-differential scanning calorimetry (TG/DSC), Fourier Transform Infrared (FT-IR), X-ray powders diffraction (XRD) and High Resolution Transmission Electron Microscope (HRTEM), respectively. The TGA/DSC, FT-IR and XRD results show that the reverse microemulsion method can effectively reduce the calcination temperature to 600 degrees C for synthesizing cubic spinel phase Mn-Co-Ni-O ceramic particles. Furthermore, the XRD and HRTEM images reveal that the particles are highly crystalline and have a narrow and uniform particle size distribution. These findings indicate that Mn-Co-Ni-O nanoparticles prepared via reverse microemulsion process meet demands for obtaining better microstructure and performance of Mn-Co-Ni-O materials.