Nickel-substituted LiMn1-xNixO2 (0 less than or equal to x less than or equal to 0.1) layered oxides have been prepared and characterized in order to examine the effect of Ni substitution on the chemical bonding nature and electrochemical property of layered lithium manganate. From X-ray diffraction and micro-Raman spectroscopic analyses, it is found that all of the nickel-substituted compounds are crystallized in an alpha -NaFeO2-type layered structure with monoclinic symmetry. The electrochemical measurements demonstrate that the replacement of Mn with Ni gives rise to only a slight improvement in electrochemical performance, illustrating the fact that Ni substitution is not so effective in reducing the capacity loss of layered lithium manganate. According to X-ray absorption spectroscopic (XAS) analyses at the Mn and Ni K edges, it is evident that the oxidation state of manganese in LiMn1-xNixO2 is increased by substituting the trivalent manganese ion with a divalent nickel ion, which leads to the reduction of the Jahn-Teller (JT) distortion around the manganese ion. However, the XAS results on the delithiated/relithiated LiMn1-xNixO2 compounds reveal that, regardless of Ni content, the irreversible transition to spinel structure is caused by Li deintercalation - intercalation reactions because of an incomplete fixation of the nickel ion accompanied by displacement of the manganese ion. On the basis of the present experimental findings, it can be concluded that the capacity loss of layered lithium manganate during the cycling process is primarily due to the migration of manganese, rather than the presence of the JT distortion.