A diffusion assembly includes a diffuser, an optical film and at least one hanging element. The optical film includes at least one hole inserted by the hanging element. The hanging element is located on the diffuser for engaging with the at least one hole of the optical film. A backlight module comprises a frame, a diffuser and an optical film. The frame comprises at least one hanging element. Both the diffuser and optical film comprise at least one hole for connecting to the hanging element on the frame.