The present invention discloses an energy efficient network communication device comprising: a media-access-controller for outputting a transmission-end low power idle (LPI) indication and receiving a reception-end LPI indication; a media-independent-interface for generating a transmission-end LPI signal according to the transmission-end LPI indication, and generating the reception-end LPI indication according to a reception-end LPI signal; and a physical-layer-circuit, coupled to several pairs of transmission lines, for converting the transmission-end LPI signal into a transmission signal to send it to a reception end for requesting an LPI mode and receiving a reception signal from the reception end to convert it into the reception-end LPI signal. Said physical-layer-circuit uses some of the several pairs of transmission lines for transmission and reception when keeping the other pairs of transmission lines unused to save power; furthermore, the physical-layer-circuit can enter the LPI mode from an idle mode for additional power saving.