A backlight unit, comprising: a frame having a bottom area and a sidewall area extended from the bottom area; a substrate on the frame; a reflection sheet positioned on the frame and the substrate, the reflection sheet having a hole; and an optical assembly mounted on the substrate and in the hole of the reflection sheet, wherein the optical assembly includes: a light source mounted on the substrate; and a lens disposed above the light source and coupled to the substrate, wherein the lens comprises: a top surface having a circular section shape; a bottom surface facing the top surface; a side surface connecting the top surface and the bottom surface; a plurality of extension parts extended from the side surface toward an outside of the lens in a radial direction of the bottom surface; and a plurality of mounting parts protruding from the bottom surface towards the substrate and fixed to the substrate, wherein a number of the plurality of extension parts corresponds to a number of the plurality of mounting parts, wherein at least one of the plurality of extension parts overlaps the substrate and at least one of the plurality of extension parts does not overlap the substrate, wherein the at least one of the plurality of extension parts not overlapping the substrate overlaps the reflection sheet around the hole of the reflection sheet to apply pressure in a Z-axis direction, which is the direction perpendicular to the bottom area of the frame, so that the reflection sheet closely adheres to the frame. a frame having a bottom area and a sidewall area extended from the bottom area; a substrate on the frame; a reflection sheet positioned on the frame and the substrate, the reflection sheet having a hole; and an optical assembly mounted on the substrate and in the hole of the reflection sheet, wherein the optical assembly includes: a light source mounted on the substrate; and a lens disposed above the light source and coupled to the substrate, wherein the lens comprises: a top surface having a circular section shape; a bottom surface facing the top surface; a side surface connecting the top surface and the bottom surface; a plurality of extension parts extended from the side surface toward an outside of the lens in a radial direction of the bottom surface; and a plurality of mounting parts protruding from the bottom surface towards the substrate and fixed to the substrate, wherein a number of the plurality of extension parts corresponds to a number of the plurality of mounting parts, wherein at least one of the plurality of extension parts overlaps the substrate and at least one of the plurality of extension parts does not overlap the substrate, wherein the at least one of the plurality of extension parts not overlapping the substrate overlaps the reflection sheet around the hole of the reflection sheet to apply pressure in a Z-axis direction, which is the direction perpendicular to the bottom area of the frame, so that the reflection sheet closely adheres to the frame. a light source mounted on the substrate; and a lens disposed above the light source and coupled to the substrate, wherein the lens comprises: a top surface having a circular section shape; a bottom surface facing the top surface; a side surface connecting the top surface and the bottom surface; a plurality of extension parts extended from the side surface toward an outside of the lens in a radial direction of the bottom surface; and a plurality of mounting parts protruding from the bottom surface towards the substrate and fixed to the substrate, wherein a number of the plurality of extension parts corresponds to a number of the plurality of mounting parts, wherein at least one of the plurality of extension parts overlaps the substrate and at least one of the plurality of extension parts does not overlap the substrate, wherein the at least one of the plurality of extension parts not overlapping the substrate overlaps the reflection sheet around the hole of the reflection sheet to apply pressure in a Z-axis direction, which is the direction perpendicular to the bottom area of the frame, so that the reflection sheet closely adheres to the frame. a top surface having a circular section shape; a bottom surface facing the top surface; a side surface connecting the top surface and the bottom surface; a plurality of extension parts extended from the side surface toward an outside of the lens in a radial direction of the bottom surface; and a plurality of mounting parts protruding from the bottom surface towards the substrate and fixed to the substrate, wherein a number of the plurality of extension parts corresponds to a number of the plurality of mounting parts, wherein at least one of the plurality of extension parts overlaps the substrate and at least one of the plurality of extension parts does not overlap the substrate, wherein the at least one of the plurality of extension parts not overlapping the substrate overlaps the reflection sheet around the hole of the reflection sheet to apply pressure in a Z-axis direction, which is the direction perpendicular to the bottom area of the frame, so that the reflection sheet closely adheres to the frame.