Provided is a field emission device, and more particularly, a field emission back light unit which makes an interconnection connected with an external electrode simple and capable of local dimming. To this end, a cathode structure for the field emission back light unit includes a plurality of data electrodes formed on a cathode substrate and spaced apart from one another, an insulating layer formed on the data electrodes, and having exposure regions exposing the predetermined data electrodes, cathode electrodes formed on the insulating layer and electrically connected with the data electrodes through the exposure regions, and at least one field emitter formed on the cathode electrodes, wherein a cathode block is defined based on the cathode electrodes electrically isolated from one another, and brightness of each cathode block can be controlled according to current supplied through the data electrode.