A lens for an LED backlight module includes a main lens body and a diffusion structure consisting of a plurality of micro lenses. The main lens body includes a light incident face for receiving light from an LED light source and a light exit face opposite to the light incident face thereof. Light leaves the lens from the light exit face. The micro lenses of the diffusion structure are formed on the light exit face of the main lens body. Each micro lens is annular and has a generally triangular cross section. Each micro lens includes a first light scattering surface and a second light scattering surface intersecting with the first light scattering surface. The micro lenses are arranged in a series of concentric circles with regard to a center of the lens. A backlight module source incorporating the lens and the LED light source is also provided.