To construct a recycling process for spent lithium-ion batteries, the hydrothermal leaching by organic acid of lith-ium-ion battery positive electrode materials and spent lithium-ion batteries was examined by treatment with citric acid (0.4‒2.0 mol/dm3) at 80‒200°C for 5‒60 min. Leaching efficiencies of Li, Co and Ni were found to increase with increasing reaction temperature and time. The leaching efficiency of Mn was more than 90% under mild conditions of 110°C for 5 min, but it decreased as the reaction temperature increased, and a white solid precipitated. However, when the temperature was further increased, the leaching efficiency increased again. When the effect of reaction temperature on leaching efficiency was investigated with spent lithium-ion batteries, the positive electrode elements Li, Co, Ni and Mn were all leached. Furthermore, it was suggested that in the leaching of spent lithium-ion batteries, the crystal struc-ture of the positive electrode elements was changed by the pretreatment, which affected the leaching mechanism.