Spent lithium ion batteries are a secondary resource of metals. To recover Li and Co ions into water, hydrothermal reactions were performed to leach cobalt and lithium from LiCoO2 at 150-300 degrees C for 10-60 min with 0.4 M of various acids. Sulfuric acid, nitric acid, and citric acid were introduced as leaching reagents. For 0.4 M of citric acid aqueous solution, over 90% of Li and Co ions could be leached at 150 degrees C within 10 min. Reduction of cobalt ion (Co3+ to Co2+) is considered to be necessary for dissolution of Co ion into water solution and citric acid acted reductant. Formation of Co citrate, which was detected by UV-vis spectroscopy, was the reason why citric acid was effective for leaching LiCoO2 with hydrothermal water.