An optical diffusion device applied in a backlight module includes a plate. Multiple light sources are disposed on one side of the plate. Multiple optical microstructures provided with a longer axis and a shorter axis are disposed on the plate. The longer axis of each optical microstructure is approximately arranged in parallel with a direction extending from the light source, or, in other words, the shorter axis is arranged approximately crossing the direction extending from the light source. A diffusion effect is provided to each light source through the shorter axis of the optical microstructure to increase optical radiant energy among the light sources and to eliminate the dim and dark regions among the light sources for increasing general luminance of the backlight module.