An illumination device includes a rectifier circuit, M light-emitting modules, and a control module. The rectifier circuit has a positive output terminal and a negative output terminal, and generates a driving voltage between the positive output terminal and the negative output terminal according to an input power. The M light-emitting modules are coupled between the positive output terminal and the negative output terminal. Each of the M light-emitting modules has a conduction voltage, and includes a light-emitting unit that includes at least one light-emitting diode. The control module is coupled between the rectifier circuit and the M light-emitting modules, and controls the M light-emitting modules to dynamically form S light-emitting diode strings coupled in parallel with each other. A number of the light-emitting units in each of the S light-emitting diode strings is N, in which S×N=M, where M, S, N are positive integers.