An illuminated keyboard is equipped with a thin key module which includes a support frame with multiple keycap installation apertures and multiple keycaps corresponding to the keycap installation apertures. Each keycap has at least three movable arms each is connected between the first wall of one side of the keycap and the second wall of the keycap installation aperture. The support frame, keycaps and movable arms are integrally formed by injection. The movable arm has a first connecting section connected to the keycap and a second connecting section connected to the first connecting section at a first bend spot. The second connecting section is extended between the keycap and support frame and connected to the keycap installation aperture. In the illuminated keyboard, light is generated by a lighting unit and transmitted through a light guide plate or light guide circuit board to project to the keycaps for illumination.