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 microstructures and at least a lamp-receiving portion. The bottom surface is opposite to the light output surface. The microstructures are formed on the light output surface and the bottom surface respectively. Each microstructure includes at least three side surfaces connected with each other, a transverse width of each side surface decreasing along a direction away from the base surface of the microstructure. The lamp-receiving portion is defined in the bottom surface. A backlight module using the present optical plate is also provided.