An optical plate (32) includes a transparent plate (321) and a light diffusion layer (322). The transparent plate includes a light output surface (3211), a light input surface (3213) opposite to the light output surface, and a plurality of spot-shaped depressions (3215) at the light input surface. The light diffusion layer is coated on the light input surface and the spot-shaped depressions, and covers the light input surface completely. The light diffusion layer includes transparent resin matrix material, and first and second light diffusion particles dispersed in the transparent resin matrix material uniformly. A refractive index of the second light diffusion particles is greater than that of the first light diffusion articles. A backlight module (30) using the present optical plate is also provided. The backlight module using the optical plate can have a thin body with a good optical performance.