A thin illuminated keyboard comprises at least one keycap, an upper light guide plate and a lower light guide plate. The upper and lower light guide plates have respectively a first light guide layer and a second light guide layer opposing each other. The first and second light guide layers have respectively a first circuit layer and a second circuit layer laid on the opposing surfaces thereof. The first and second light guide layers also have respectively a first mask layer and a second reflection layer on the sides opposite to the first and second circuit layers to form an optical passage therebetween. The first mask layer has at least one light penetration zone receiving light from the optical passage. The light is transmitted and confined in the optical passage between the first and second light guide layers, and condensed to project to the keycap through the light penetration zone.