A protective circuit of a battery pack and a method of operating the protective circuit prevents the battery cells from being overcharged by directly detecting overcharge or detecting disconnection of sensing wires, which sense the voltage of battery cells, and instantly shuts off the high current being applied to the battery cells when the sensing wires are disconnected. The protective circuit includes at least one battery cell, and a cell voltage detecting unit adapted to detect the voltage of the battery cell. The protective circuit also includes a switch unit, and a control unit adapted to receive a signal from the cell voltage detecting unit and generate a predetermined control signal based on the signal of the cell voltage detecting unit in order to turn off the switch unit when thebattery cell is overcharged or the sensing wire is disconnected.