Abstract: A brake light system for bicycles includes a light means fixedly mounted on a bicycle and a pair of brake assemblies each having a metallic bracket, a insulating rubber member mounted on the first metallic bracket, and a conductive rubber member mounted on the insulating rubber member, the metallic bracket being provided with a brake post extending outwardly to engage with one of the arms of a cantilever, the conductive rubber member being electrically connected with the light device via a electrical wire, whereby when the cantilever brake is activated, a braking signal will be transmitted to a controlling circuit through the conductive rubber member thereby turning on the light device.