Disclosed are an optical sheet, which includes a prism pattern having condensing axes crossing each other in diagonal directions with respect to an arrangement direction of light sources, thereby achieving an enhanced contrast ratio without a reduction in viewing angle and an increase in costs, and a backlight unit using the same. The optical sheet includes a plurality of crossed prism patterns each having a first condensing axis and a second condensing axis crossing each other in diagonal directions with respect to an arrangement direction of a light source.