An exemplary optical plate (23) includes a transparent plate portion 231, a plurality of brightness enhancement structures (232) and a light diffusion layer (233). The transparent plate portion has a light input interface (2311), and a light output surface (2312) opposite to the light input interface. The brightness enhancement structures are formed at the light output surface. The light diffusion layer are formed on the light input interface, which includes a transparent resin matrix material and a plurality of first light diffusion particles and a plurality of 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. An exemplary method for manufacturing the optical plate is also provided. The optical plate and an associated backlight module (20) can increase the light energy utilization ratio.