Abstract: A shut down mechanism for handheld devices is disclosed. The shut down mechanism for handheld devices consists of a fixing part, a magnetic object and a magnetic sensor. The fixing part is disposed on a handheld device for fixing batteries of the handheld device, the magnetic object is arranged on the fixing part and the magnetic sensor is set on the handheld device. When the fixing part is rotated for replacing batteries, the magnetic sensor detects magnetic power from the magnetic object and sends a signal to a central processing unit in the handheld device for running shut down procedure. Thus when users replace batteries of handheld devices without turning off the handheld device, the shut down procedure is executed automatically so as to avoid malfunction or damage of the handheld device.