An exemplary prism sheet includes a transparent main body. The main body includes a first surface and a second surface. The first surface and the second surface are on opposite sides of the main body. The first surface and the second surface define a plurality of first micro-depressions and a plurality of second micro-depressions respectively. Each first micro-depression is defined by four connecting curved inner sidewalls. A transverse width of each curved inner sidewall of each first micro-depression progressively decreases with increasing distance from the first surface. Each second micro-depression is defined by four connecting inner sidewalls. A transverse width of each inner sidewall of each second micro-depression progressively decreases with increasing distance from the second surface. A backlight module using the present prism sheet is also provided.