An organic electrolytic solution and a rechargeable lithium battery comprising the same is provided. The organic electrolytic solution contains a lithium salt and an organic solvent mixture. The organic solvent mixture is comprised of a solvent with high permittivity, a solvent with a low boiling point, and a cyclic organic compound having at least an oxy-carbonyl group and having a ring structure of 6 units or more. The organic electrolytic solution helps to increase reduction decomposition stability in the lithium battery using the same. As a result, the irreversible capacity of the lithium battery at a first cycle decreases and charge/discharge efficiency and lifespan of the lithium battery increases. In addition, the battery thickness is maintained within a specific range at room temperature after formation and standard charging, which improves the reliability of the lithium battery.