A scheme for the photonic generation of frequency-tunable millimeter wave and terahertz wave signals based on a highly flat optical frequency comb is proposed and demonstrated experimentally. The frequency comb is generated using two cascaded phase modulators (PMs) and an electro-absorption modulator (EAM). The frequency comb covers a 440-GHz frequency range, with 40-GHz comb spacing and less than 2-dB amplitude variation. By filtering out two of the comb lines with 50 dB out of the band suppression ratio, high frequency-purity and low phase noise millimeter wave or terahertz wave signals are successfully generated, with frequencies ranging from 40 to 440 GHz.