Abstract: Disclosed is a caller ID device with a power saving function according to the invention. The caller ID device, electrically coupled to a pair of telephone lines together with a telephone, includes a CAS receiver, a current limiting circuitry, an FSK receiver, an FSK mark or space tone detector, a controller and a DC battery. At an off-hook state, the current limiting circuitry can directs an operating current from the pair of telephone lines to the CAS receiver through the telephone for power supply. At an on-hook state, the FSK mark or space tone detector can detect an FSK mark or space signal from the pair of the telephone lines and then output an FSK mark or space detecting signal in response to the received FSK mark or space signal, thereby enabling the controller to activate the FSK receiver ready to receive an FSK demodulate signal from the pair of the telephone lines.