The self-discharge mechanism of LiNi1/3Co1/3Mn1/3O2 cathode for lithium ion battery at high potential (4.5 V) has been understood through physical and electrochemical characterizations including charge/discharge test, electrochemical impedance spectroscopy (EIS), inductively coupled plasma atomic emission spectrometer (ICP-AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). It is found that the charged LiNi1/3Co1/3Mn1/3O2 cathode to 4.5 V suffers seriously self-discharge. After storage for 8 days, the potential of the cathode charged to 4.2 V remains stable, while that of the charged cathode to 4.5 V decreases from 4.5 to 1.0 V, The characterizations, from SEM, TEM, ICP-AES, and XRD, demonstrate that this self-discharge results from the interaction between charged LiNi1/3Co1/3Mn1/3O2 and electrolyte, which causes the dissolution of transition metals from LiNi1/3Co1/3Mn1/3O2 and the successive decomposition of the electrolyte.