A diffusion-plates assembly and a direct-type backlight module assembly are provided. The diffusion-plates assembly includes a first diffusion plate and a second diffusion plate. There are pluralities of first microstructures on the emergent surface of the first diffusion plate, the first microstructures is for condensing light. The thickness of the first diffusion plate is above 0.4 mm, and the transmittance of the first diffusion plate is above 90%. The second diffusion plate is placed over or under the first diffusion plate, The thickness of the second diffusion plate is above 0.4 mm, and the transmittance of the second diffusion plate is 50.about.90. Said diffusion-plates assembly is used in the direct-type backlight module assembly.