A back light unit including a light emitting device assembly is disclosed. The light emitting device assembly includes a light emitting device module having a light emitting device, a light guide plate having a light incidence part disposed adjacent to the light emitting device module so that light generated from the light emitting device is incident upon the light incidence part and a light emission part from which the incident light is emitted, a reflective sheet provided at one side of the light guide plate, and a light adjustment part extending from the reflective sheet for adjusting a reflection amount of the light emitted from the light guide plate.