An optical sheet, a backlight unit including the optical sheet, and a liquid crystal display including the backlight unit may be provided. The optical sheet may include a reflective polarizing film, a base film on one surface of the reflective polarizing film, and a projection portion on the base film. The projection portion may include a plurality of diffusion particles, a plurality of peaks, a plurality of valleys formed from the plurality of peaks, and a base portion between the base film and the peaks and the valleys. A height of the base portion is approximately 5% to 50% of a height of one of the peaks.