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 enclosing V-shaped protrusions, a plurality of microstructures and a lamp-receiving portion. The light output surface and the bottom surface are on opposite sides of the optical plate. The enclosing V-shaped protrusions are formed on the bottom surface. The microstructures are formed on the light output surface. Each microstructure has circular cross-section taken along a plane parallel to its base surface thereof, a diameter of the circular cross-section decreasing along a direction away from its base surface thereof. The lamp-receiving portion is defined in at least one of the light output surface and the bottom surface. A backlight module using the present optical plate is also provided.