Stimulated Brillouin scattering (SBS) is a key problem with the increasing power of fiber transmission systems. In this letter, a frequency-modulated fiber laser with an ultra-narrow linewidth is chosen as a light source. The SBS threshold is increased from 4.1 to 6.2 mW at 13-MHz frequency modulation amplitude for a 50-km G652 fiber. We also show that the SBS threshold increases with not only the frequency modulation amplitude, but also the modulation frequency. The modulation frequency should be high enough for effective modulation.