Provided is a method of saving power in a passive optical network (PON) system including an optical line terminal (OLT) and a plurality of optical network units (ONUs), the OLT including an optical transceiver to communicate with at least one ONU through an optical line, and a controller to control the optical transceiver to transmit an upstream bandwidth map to the ONU at a predetermined transmission interval, wherein the transmission interval is determined based on a desired upstream data service delay time.