An edge light backlight device includes a light guide plate having a base portion and microstructures, LEDs, and a reflective plate. The base portion has a light emitting side, a rear side, and a light incident side. The microstructures are formed on one of the light emitting side and the rear side and each of which extends from the light incident side in a longitudinal direction. Each of the microstructures includes a curved surface with a radius of curvature (R). The light guide plate has a thickness (T). R/T ratio ranges from 0.04 to 0.15 and each of the microstructures has a height ranging from 20 μm to 300 μm so that the light guide plate has a performance of local lighting ranging from 1% to 40%.