The sol-gel method is applied to prepare nano-sized LMNCO lithium-rich layered oxides by adding of composite organic acid (citric acid & tartaric acid). The effect of composite organic acid on the microstructure of Li1.2Mn0.54Ni0.13Co0.13O2 (LMNCO) Li-rich cathode material is explored in this work. The results confirm the existence of interactions between the composite organic acid molecules and the precursors, and formation of stronger space steric effect. The stronger space steric effect can effectively prevent the growth of precursor particles in the presintering process at 550 °C for 5 h, which eventually contributed to the fabrication of nano-sized LMNCO. According to EIS analysis, the synthesized LMNCO has low charge transfer resistance of 135.4 Ω. Furthermore, it shows excellent electrochemical performance with a discharge capacity of 263.1 mAh g−1 at 0.1 C and 187.9 mAh g−1 at 5 C.