An exemplary optical plate includes a plurality of transparent plate units. The transparent plate unit includes a first surface, a second surface, a plurality of conical frustum protrusions, a diffusion layer and a lamp-receiving portion. The second surface is opposite to the first surface. The conical frustum protrusions are formed at the first surface. Each conical frustum protrusion comprises two conical frustums. The diffusion layer is formed at the second surface. The lamp-receiving portion is defined in one of the first surface and the second surface. A backlight module using the optical plate is also provided.