An illuminating device includes a light source and a light diffusing plate. The light diffusing plate includes a light pervious substrate and a light diffusing structure. The light pervious substrate has a lower surface adjacent to the light source and an upper surface at an opposite side of the substrate to the lower surface. A through hole is defined in the substrate to expose a central portion of the light source. The light diffusing structure is formed at the upper surface and includes a plurality of first lens arrays radially extending from the through hole and a plurality of second lens arrays each arranged between two adjacent first lens arrays.