An optical network unit (ONU) having a power saving function for reducing power consumption in an optical network and a power saving method thereof are provided. The ONU includes a first signal transmitter-receiver unit configured to transmit and receive a signal to and from an optical line terminal, a second signal transmitter-receiver unit configured to transmit and receive a signal to and from customer premises equipment through multiple communication ports, a signal processing unit configured to control a signal to be transmitted and received through the first signal transmitter-receiver unit and the second signal transmitter-receiver unit, and a processor configured to control powers of the first signal transmitter-receiver unit and the signal processing unit according to whether data traffic is received through the plurality of communication ports.