A backlight unit comprising: a plurality of light emitting units; and a reflector including a plurality of reflector units formed to have spaces for receiving the plurality of light emitting units, respectively, wherein each light emitting unit includes: an LED package mounted on a substrate, the LED package having a bottom surface facing the substrate and a top surface opposite to the bottom surface; wherein each space is formed by a wall which surrounds the outer circumference of a corresponding LED package and is formed in an expanding shape which is larger at an end that is away from the substrate; and characterized in that each light emitting unit further includes: a light diffusion layer comprising a bottom surface, an outer circumferential surface and a top surface, the light diffusion layer including a groove formed in its top surface, the light diffusion layer being formed on the top surface of the LED package and wherein the outer circumferential surface of the light diffusion layer faces an inner surface of the reflector unit, and is surrounded by the corresponding wall of the reflector unit. a plurality of light emitting units; and a reflector including a plurality of reflector units formed to have spaces for receiving the plurality of light emitting units, respectively, wherein each light emitting unit includes: an LED package mounted on a substrate, the LED package having a bottom surface facing the substrate and a top surface opposite to the bottom surface; wherein each space is formed by a wall which surrounds the outer circumference of a corresponding LED package and is formed in an expanding shape which is larger at an end that is away from the substrate; and characterized in that each light emitting unit further includes: a light diffusion layer comprising a bottom surface, an outer circumferential surface and a top surface, the light diffusion layer including a groove formed in its top surface, the light diffusion layer being formed on the top surface of the LED package and wherein the outer circumferential surface of the light diffusion layer faces an inner surface of the reflector unit, and is surrounded by the corresponding wall of the reflector unit. an LED package mounted on a substrate, the LED package having a bottom surface facing the substrate and a top surface opposite to the bottom surface; wherein each space is formed by a wall which surrounds the outer circumference of a corresponding LED package and is formed in an expanding shape which is larger at an end that is away from the substrate; and characterized in that each light emitting unit further includes: a light diffusion layer comprising a bottom surface, an outer circumferential surface and a top surface, the light diffusion layer including a groove formed in its top surface, the light diffusion layer being formed on the top surface of the LED package and wherein the outer circumferential surface of the light diffusion layer faces an inner surface of the reflector unit, and is surrounded by the corresponding wall of the reflector unit. a light diffusion layer comprising a bottom surface, an outer circumferential surface and a top surface, the light diffusion layer including a groove formed in its top surface, the light diffusion layer being formed on the top surface of the LED package and wherein the outer circumferential surface of the light diffusion layer faces an inner surface of the reflector unit, and is surrounded by the corresponding wall of the reflector unit.