The microelectrode technique was applied to investigate the electrochemical behavior of LiNiO 2 which seriously influenced by water molecules. Cyclic voltammograms of humidified LiNiO 2 particles were compared to that of as-synthesized. As a result, a significant peak-potential shift was observed, which denotes that the reaction product caused by H 2 O generates an electrochemical inactive material on the particle which prevents the Li + insertion/extraction. The reaction product was considered to be NiO, and deterioration property has also been discussed.