An exemplary optical plate (20) includes a transparent plate (21) and a light diffusion layer (22). The transparent plate includes a light output surface (212), a light input surface (211) opposite to the light output surface, and a plurality of elongated recessed portions (213) formed at the light input surface. The light diffusion layer is coated in the elongated recessed portions. The light diffusion layer and the light input surface cooperatively define a flat surface. A backlight module using the present optical plate is also provided. The present backlight module using the optical plate can have a thin body with a good optical performance.