A protection circuit for a lithium battery pack that serves as a power source of a power tool is arranged in the lithium battery pack and connected among a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool. During a discharging process, the protection circuit detects battery voltage and temperature and when abnormal events occur, the protection circuit cuts off the discharge current. The protection circuit has a simple structure, low costs, and is easy to incorporate to the currently existing pressing-control circuit of the power tool.