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