An approach of multiple-frequency millimeter-wave (mm-wave) signal generation is proposed for radio-over-fiber (RoF) system with multiple-frequency basestations (MFBSs). Two groups of orthogonally polarized signals are injected into a semiconductor optical amplifier (SOA), and subsequently ten new different wavelengths are generated via four-wave mixing (FWM) effect. At each MFBS, different wavelengths are filtered out using demultiplexer and then input to a photodiode (PD) to generate the mm-wave signals with the frequencies from 52 GHz to 68 GHz at the interval of 2 GHz. Simulation results verify that the proposed multiple-frequency generation for MFBS RoF system can work properly.