We demonstrate a tunable light-wave to millimeter-wave converting system based on optical side band heterodyne and stimulated brillouin scattering in a fiber system composed of a DFB laser, a dual-electrode external Mach-Zehnder modulator (MZM), a pumping laser, a single-mode fiber and a photo-detector. The MZM is modulated by a baseband digital signal in one arm and by a microwave in another arm to generate multiple side modes around the central wavelength. One of the side modes is amplified by SBS gain caused by the pump laser so that the heterodyne in photo-detector yields a baseband-modulated millimeter wave tunable by the pump laser.