An illuminating device installed on a truck's wheels includes a hubcap having a positioning base and at least one fixing base. The positioning base is provided with a chamber and two notches formed in its one side wall. The fixing base has a recess, a through hole bored in the recess to communicate with an outside of the hubcap, and two recessed slot. A generator is fixed at an inside center of the hubcap, provided with two conducting blades extending into the chamber of the positioning base. At least a light is fitted in the through hole of the hubcap and connected with the conducting blades by conducting lines. By means of rotation or vibration of the wheels, the generator can create electromotive force to supply the light with power to make rotary brilliant brightness so as to achieve a warning effect for traffic security.