We have proposed and experimentally demonstrated a new radio-over-fiber system technique generating optical millimeter-waves with central carriers suppressed by simply tuning the modulation index of an optical phase modulator without requiring any complicated bias control circuits or narrowband optical filters. Error-free transmission of the generated 40-GHz optical mm-wave with 2.5-Gb/s data over 10-km single mode fiber (SMF-28) and 3-m air distance was achieved. The experimental results were analyzed and compared with the traditional method that requires a specific carrier suppression optical filter. The power penalty caused by the crosstalk between wavelength division multiplexed (WDM) channels was experimentally measured and studied and was about 4 dB at a bit error rate (BER) of 10-9. The filter bandwidth requirement, achievable carrier suppression ratio, and harmonics fluctuation due to fiber dispersion were also theoretically analyzed.