An optical diffusion device applied in a backlight module includes a plate, multiple light sources being disposed on one side of the plate; multiple optical microstructure provided with a longer axis and a shorter axis being disposed on the plate; a direction of the longer axis of each optical microstructure being approximately arranged in parallel with a direction extending from the light source, or a direction of the shorter axis being arranged approximately crossing a direction extending from the light source; better diffusion effect being provided to each light source through the shorter axis of the optical microstructure to increase optical quantum among light sources thus to eliminate the dim and dark regions among light sources for increasing general luminance of the backlight module.