An exemplary optical plate includes at least one transparent plate unit. The transparent plate unit includes a light output surface, a bottom surface, a plurality of spherical depressions, a diffusion layer, and at least one lamp-receiving portion. The bottom surface is opposite to the light output surface. The diffusion layer is formed on the light output surface. The spherical depressions are formed at the bottom surface. The lamp-receiving portion is defined in the bottom surface. A backlight module using the present optical plate is also provided.