An object of the present invention is to provide a substrate film for optical sheets, an optical sheet and a backlight unit which can markedly enhance utilization efficiency of rays of light and dramatically increase luminance and which are suited for immediate beneath type liquid crystal display modules and the like. The present invention is characterized by a substrate film for optical sheets formed into a rectangular shape and made of a resin, in which the substrate film for optical sheets has an optical anisotropy, and the absolute value of an angle of the crystal orientation with respect to the short side orientation is π/8 or greater and 3π/8 or less. The retardation value of the substrate film is preferably 70 nm or greater and 320 nm or less. Matrix resin constituting the substrate film is preferably polyethylene terephthalate or polycarbonate. The optical sheet of the present invention includes the substrate film for optical sheets, and an optical layer laminated on one face of the substrate film for optical sheets. The liquid crystal display module of the present invention has a liquid crystal display element, the optical sheet and an immediate beneath type backlight.