The backlight unit may include a plurality of optical assemblies, which each include a light emitting module having a substrate, a plurality of light emitting devices on a top surface of the substrate, and a connector provided on a bottom surface of the substrate and electrically connected to a power supply unit. The optical assembly may also include a light guide plate including a first part to receive the light and a second part to output the light through a top surface. The optical assembly may also include a side cover that may fix the light emitting module and a portion of the first part. The side cover may have a plurality of connector holes.