A powerline network system having data relay function includes a plurality of power monitoring zones mutually connected with a powerline network and at least one repeater. Each of the power monitoring zones has at least one power monitoring device. The repeater is connected with each power monitoring device in two adjacent power monitoring zones through powerlines and has a first and a second relay units. The first and second relay units employ two different sets of transmitting and receiving bands to fetch data contained in powerline signals transmitted by each power monitor in adjacent power monitoring zones. The data are received by a relay unit are modulated with different transmitting band and then transmitted to powerlines of another power monitoring zone. The repeaters are not required to be serially connected to the powerlines, and incorrect data read can be also eliminated.