Abstract: This invention relates to a flat prism sheet for backlight units and a method of manufacturing the same, and particularly, to a flat prism sheet, which has flat upper and lower surfaces and prism-shaped protrusions formed therein, and to a method of manufacturing the same. The flat prism sheet for backlight units according to this invention includes a lower layer, a first intermediate layer formed on the upper surface of the lower layer and having a plurality of prism-shaped protrusions formed parallel to each other, a second intermediate layer formed on the upper surface of the first intermediate layer, and an upper layer formed on the upper surface of the second intermediate layer, wherein an air layer is formed between the lower surface of the second intermediate layer and the valleys of the prism-shaped protrusions of the first intermediate layer.