A backlight module for a liquid crystal display includes a reflective sheet, at least one light source above the reflective sheet, and a lens above the light source. The lens has a light-permeable surface at the bottom, a light-reflecting surface formed on the top of the lens, two side surfaces connecting the light-reflecting surface and the light-permeable surface, and a diffusion surface formed at the center of the light-reflecting surface. Profiles of the light-permeable surface and the light-reflecting surface are concave. The diffusion surface is convex. The lens of the invention allows light emitted by the light source to diffuse more uniformly.