A light guiding diffuser used for assembling with a light emitting diode module to form an illuminating light source. The light guiding diffuser includes an elliptical ring-shaped base portion and a convex portion extended from a surface of the base portion and formed a space with the base portion for accommodating the light emitting diode module. The convex portion has a cross section which has a minimum thickness in a center thereof and the thickness of the cross section continuous increases from the center toward the base portion.