A diffusion plate (20) includes at least two diffusion units (21), each diffusion unit including at least one connecting portion (23). The connecting portions (23a, 23b) of each two adjacent diffusion units mate with each other, thereby connecting all the diffusion units together. The number of diffusion units may be increased or decreased according to the size of a liquid crystal display to be produced. This eliminates the need to alter the production means for differently sized backlight modules, thereby reducing costs and increasing production efficiency. In another embodiment, a diffusion plate (50) includes at least two diffusion units (51). Each diffusion unit includes at least one connection protrusion (53a). At least one interconnecting member (53) is disposed between each two adjacent diffusion units. The interconnecting member and the connecting protrusions mate with each other, thereby connecting all the diffusion units together. A related backlight module (500) is also provided.