The effect of the particle size of LiCoO2 on the operating temperature-dependent performance of lithium batteries is investigated. The LiCoO2 particle size is successfully controlled by modifying the powder preparation process and well-controlled nanocrystalline LiCoO2 powders are obtained. It is found that the discharge capacity of a cell made with the nanocrystalline powder is slightly lower than that of cells made with micron-sized powders. On the other hand, the cycle performance of the cell using nanocrystalline LiCoO2 powder is consistent over the selected operating range of temperature (−15 to 60°C) without deterioration. It is concluded that the smaller size of the particles contributes to the enhancement of the reliability by increasing the specific surface-area for intercalation sites, and by enhancing the resistance to mechanical failure especially at higher temperatures.