The nanoparticles based on nickel-manganese oxide and carbon-coated LiNi0.5Mn1.5O4 are synthesized by flame spray pyrolysis technology with controlled particle sizes. The structural properties of nanoparticles are characterized by X-ray diffraction and high-resolution electron microscopy. It is observed that the higher surface tension of precursors in the flame spray pyrolysis setup increases the particle sizes. The post annealing treatment significantly enhances the crystallinity of nanoparticles due to the favorable oxidation process and the structure conversion from NiMn2O4 to NiMnO3. In addition, the solid-state reaction of as-prepared NiMn2O4 results in the LiNi0.5Mn1.5O4 nanoparticles which can be applied to the cathode in Li-ion batteries. The carbon-coated LiNi0.5Mn1.5O4 nanoparticles are also made by additional carbonization reaction. The control of surface tension of precursors in the flame spray pyrolysis process is expected to result in the small size of nanoparticles, which can result in high cyclic stability and high capacity for Li-ion batteries.