A novel scheme to generate optical millimeter-wave (mm-wave) signals by using single-sideband (SSB) modulation technique was proposed and experimentally demonstrated. Odd order sidebands were suppressed by proper adjusting the direct current (DC) bias of signal-arm intensity external modulator in central station. One of the second order sidebands was filtered out and SSB modulation was realized. The mm-wave generated by SSB modulation can be transmitted over 148 km in theory. In experiment, a 2.5 Gbit/s data and an 10 GHz RF clock were mixed. The mixed signals were applied to drive a single-arm intensity modulator to generate 40 GHz double sideband signals. Then a filter is used to get a 20 GHz SSB signal. The effect of optical carrier-to-sideband ratio (CSR) on downstream transmission in radio-over-fiber (ROF) system was analyzed and the performance of the ROF system can be significantly improved when CSR is 0 dB.