A backlight assembly (100) for providing bright and uniform light beams to illuminate an LCD panel includes a light source (120), a reflector (130), and a diffusion sheet (150). The light source is positioned between the reflector and the diffusion sheet, and the diffusion sheet has at least one diffraction grating (151) formed on a surface thereof facing the light source. The backlight assembly has a simple configuration for providing bright and uniform light beams to illuminate an LCD panel. Furthermore, the diffusion sheet is made of a transparent piezoelectric material. When an electric field is applied on the diffusion sheet by a controlling circuit, the diffusion sheet is induced to deform. Thus the distribution of light intensity can be modulated according to need.