A backlighting device includes a light guide plate having a first side and a second side opposite to the first side, and a point light source to generate light incident on the first side of the light guide plate. The light guide plate has a plurality of longitudinal diffraction microstructures which are arranged and oriented according to the location of the point light source. The microstructures are varied gradually at least in size, position, density, or shape relative to the location of the point light source. When light is incident on the light guide plate from the point light source, the microstructures, which are varied gradually at least in size, position, density, or shape, function to regulate and adjust the illuminance of bright and dark areas in the light guide plate so as to provide uniform brightness.