A radio over fiber (ROF) system to generate optical millimeter wave with four time frequency of the local oscillator frequency using two-cascaded intensity modulators based on optical carrier suppression scheme was investigated numerically and experimentally. In the central office, the upstream data were mixed with the radio frequency (RF) clock to generate a electrical signal, and then an external modulator was used to generate a optical millimeter with double freqnency of RF clock, after the second external modulator, the optical millimeter wave was generated with frequency quadruple. By this scheme, a 40 GHz millimeter wave was generated by the frequency mixing of the 10 GHz signal source and the 2.5 Gbit/s baseband data signal passing through the two external modulator. After transmission over single-mode fiber (SMF) 20 km, the power penality is less than 2 dB.