The oxalate co-precipitation method was used to synthesize LiNi0.5Mn0.5O2. The effects of pH on the structure, morphology, and electrochemical performance of LiNi0.5Mn0.5O2 were investigated. The crystal structures and surface morphologies of the oxalate precursor and LiNi0.5Mn0.5O2 obtained at pH=4.0, 5.5, 7.0, 8.5 were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. The electrochemical performance of LiNi0.5Mn0.5O2 was evaluated by galvanostatic charge/discharge tests. Results show that the LiNi0.5Mn0.5O2 obtained at pH=7.0 has a smaller particle size, more uniform distribution, better layered characteristics and a smaller degree of cation mixing. Electrochemical tests confirmed that the sample obtained at pH=7.0 had the best electrochemical performance. At 0.1C rate, its discharge capacity reached 185 mAh·g-1 at the first cycle and remained over 160 mAh·g-1 after the 20th cycle. X-ray photoelectron spectroscopy (XPS) results indicated that the oxidation states of Ni and Mn in the LiNi0.5Mn0.5O2 obtained at pH=7.0 were +2 and +4, respectively.