A novel scheme for optical millimeter-wave generation with double-sideband (DSB) modulation by external modulator in a radio-over-fiber (ROF) system was proposed. At the central station, a CW lightwave was intensity-modulated by the RF signal to generate DSB signal. After the pure optical carrier was filtered, an optical interleaver was employed to separate the spectrum of the upper sideband and the lower sideband of the generated DSB signal. After the lower sideband signal was modulated with a baseband data at 2.5 Gbit/s, it was recombined with the unmodulated signal to generate optical millimeter wave, and then transmitted to the base station over single-mode fiber (SMF). In the base station, the electrical millimeter wave signal was generated by O/E conversion. The dispersion performance of the generated mm-wave is theoretically analysed, it can immune the fading effect. The experimental results show that the downlink data signals can transmit over 50 km fiber without dispersion compensation, and the power penalty is less than 1 dBm. So the generated optical millimeter wave by proposed method can effectively reduce baseband signal degeneration caused by the time shift of the code edges because of fiber dispersion, it is suitable for long distance transmission.