The detail experimentalstudy of phase modulation for Stimulated Brillouin Scattering (SBS) suppression in opticalfiber AM-CATV externally modulated transmitter is presented in this paper. Theexperimental data show that the improvement of SBS threshold is dependent of the number ofSBS suppression source and the phase modulation index, and independent of the frequency ofSBS suppression signal. The experiments also show that single signal phase modulation canimprove the SBS threshold to 13 dBm and two signal phase modulation can improves the SBSthreshold to 18 dBm or above. It is also demonstrated in this paper that the phasemodulation has no influence on the nonlinearity in optical fiber AM-CATV externallymodulated transmission system.