A prism sheet (10) includes a substrate (12) and a plurality of prism structures (14). The substrate includes a plurality of diffusing particles (122) dispersed therein. The prism structures are at a light output side of the substrate. The diffusing particles can diffuse light beams traveling through the substrate. This configuration performs the same function as that of a diffusing sheet of a conventional backlight module. The prism structures can gather light beams to improve a brightness of a backlight module (6) incorporating the prism sheet. Thus, the backlight module can provide bright and uniform light at a low cost and while having a reduced thickness.