A novel scheme was proposed to generate sextupling-frequency and octupling-frequency optical millimeter wave by using external modulator technique. Based on the nonlinear transmission characteristics of one Mach-Zehnder modulator(MZM), the third-order or fourth-order sideband could be enhanced by adjusting the bias voltage and the modulation voltage of MZM, and thus generated successfully sextupling-frequency or octupling-frequency optical millimeter wave. Walk-off effect could be prevented by only modulating the signal on one of the third-order or fourth-order sidebands. Numerical results demonstrate that the proposed scheme generate 60 GHz millimeter wave with only 10 GHz and 7.5 GHz frequency modulated signal, which greatly reduces the frequency of the modulation signal, increases the up-conversion coefficient, and reaches the transmission distance to 160 km without increasing the power penalty distinctly. Compared with other multiple frequency techniques, the proposed system has distinct advantages of simple configuration, low dispersion effect, long transmission distance due to only using one Mach-Zehnder modulator.