An exemplary prism sheet (20) includes a transparent main body. The main body includes a first surface (201), a second surface (202) opposite to the first surface, a plurality of first micro-depressions (203) defined in the first surface, and a plurality of second micro-depressions (204) defined in the second surface. Each of the first micro-depressions is conical frustum-shaped. Each of the first micro-depressions has four inner side surfaces connected to each other. A transverse width of each inner side surface of the first micro-depressions progressively decreases with increasing distance from its bottom surface that are coplanar with one of the first surface of the transparent main body. A backlight module (200) using the present prism sheet is also provided.