A dimmable LED illumination system includes a power converting circuit and a lamp. The power converting circuit is electrically connected to a power source to convert electric power of the power source into a voltage signal. The lamp includes a DC/DC converter and a LED module, wherein the DC/DC converter is electrically connected to the power converting circuit to convert the voltage signal into a corresponding current signal. The LED module is electrically connected to the DC/DC converter to be driven by the current signal to emit light. By adjusting a voltage provided by the power converting circuit, a current provided by the DC/DC converter could be adjusted as well, which changes a luminance of the LED module.