An exemplary optical plate (30) includes a transparent plate (321) and a light diffusion layer (322). The transparent plate includes a light output surface (3213), a light input surface (3211) opposite to the light output surface, and a plurality of elongated depressions (3215) formed on the light input surface. The light diffusion layer is coated on the light input surface and filled with the elongated depressions. The light diffusion layer covers the light input surface. The tight diffusion layer includes transparent resin matrix material, first light diffusion particles 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 particles. A backlight module using the present optical plate is also provided. The present backlight module using the optical plate can have a thin body with a good optical performance.