A backlight module is provided herein, which mainly contains a casing, a light mixing plate, a diffusion plate, a number of LEDs and optical sheets. The LEDs are positioned along the light incidence planes at the sides of the light mixing plate. The casing is positioned in front of the light mixing plate with the diffusion plate at the back opening of the light mixing plate. The optical sheets are placed in turn behind the diffusion plate. As such, the thickness of the backlight module is reduced significantly.