An exemplary prism sheet includes a transparent main body. The main body includes a first surface and a second surface. The first surface is opposite to the second surface. A plurality of elongated V-shaped protrusions are formed on the first surface. A plurality of micro-depressions are defined in the second surface. Each micro-depression is defined by at least three connecting inner sidewalls. A transverse width of each inner sidewall of each micro-depression progressively decreases with increasing distance from the second surface. A backlight module using the present prism sheet is also provided.