A lock structure including an activation device, a combination lock, and a setting device is provided. The combination lock has a plurality of dials, a shaft passing through the dials, and a plurality of first gears disposed on one side of each dial. Each first gear further includes a missing tooth. One end of the shaft is connected to the activation device. The setting device has a plurality of second gears corresponding to the first gears and a setting shaft, wherein one end of the setting shaft passes through the second gears and the other end is connected to the activation device. When the activation device is activated, the second gears are driven to rotate the first gears, so that when each second gear runs free with the missing tooth of each first gear, the dials are in a predetermined position. That is, by means of the activation device and the setting device, the dials of the combination lock can automatically return to the predetermined position after opening or closing the lock.