An Electric tool with a controller for a battery pack and a control method thereof. The electric tool includes a battery pack, an operator and a controller The battery pack includes a plurality of battery cells and a protection circuit module detects and controls a state of charge and discharge of a plurality of battery cells. The operator receives power from the battery pack and converts the received power into physical energy. The controller controls the protection circuit module so as to perform a protection process of the battery pack in a state similar to over-discharge caused in the case in which the power is transferred from the battery pack to the operator and at the same time, the operator does not continuously operate.