A back light unit providing a direction controllable collimated light beam and a three-dimensional display using the same is described. A back light unit comprises: a light source for generating light; and a light direction controller for converting the light from the light source into a direction controlled collimated light beam having a refraction angle, the direction controlled collimated light beam emitted to a predetermined area to generate a three-dimensional holographic image. The back light unit can provide direction controllable collimation light having uniform brightness distribution over a large diagonal area of the spatial light modulator with thin and simple structure.