A power line adapter and its controlling method to be operated in a power-saving mode are disclosed. The power line adapter includes a power-transmitting module for generating a DC supply voltage and a data-transmitting module for transmitting data. Furthermore, the data-transmitting module at least includes a power line communication circuit and an Ethernet physical layer transceiver. The controlling method comprises steps of: monitoring a link signal generated by the Ethernet physical layer transceiver when the power line adapter is operated in an active mode; and, disconnecting the DC supply voltage to the power line communication circuit and entering the power-saving mode if the link signal indicates a disconnection over a first time period.