A battery pack of an electric power tool comprises ten lithium-ion cells. The ten lithium-ion cells are connected in series. Each lithium-ion cell has a diameter equal to or less than 18 millimeters, a length equal to or less than 65 millimeters, and an internal resistance equal to or less than 30 milliohms. Because the battery pack has a high voltage in operation and is therefore able to supply large current, the battery pack is preferably configured incapable of being used in a conventional electric power tool that cannot operate under such a large current.