In this paper, we have reported a radio-over-fiber (ROF) system with 18-tuple frequency optical millimeter-wave using external modulator and four-wave mixing (FWM) in semiconductor optical amplifier (SOA). Signal lightwave and pump with frequency interval of 6fRF are generated by adjusting the parameters of the Mach-Zehnder modulator (MZM). After FWM in SOA and filtering, 18-tuple frequency millimeter-wave signal is generated. Based on the proposed scheme, a 90 GHz optical millimeter-wave is generated only using 5 GHz RF by numerical simulation. The simulation results show that our proposed scheme can provide 20 km single-mode fiber (SMF) transmission. In addition, the impact of SOA’s current on the sideband ratio (SR) and the system performance are also studied.