In the conventional high-frequency millimeter wave system, the high-order sidebands can be directly ignored when the sideband suppression ratio is relatively large. However, for a small sideband suppression ratio, the big power of a high-order sideband obviously influences the generation of millimeter waves. The effects of high-order sidebands on the generated high-frequency millimeter waves are mainly demonstrated in this paper. In addition, an eightfold-frequency millimeter wave is obtained based on even-order sideband suppression by the cascaded Mach-Zehnder modulators (MZMs). In an actual optical millimeter wave system, the redundant low-order sidebands usually occur after the modulation of high-order sidebands, which makes the power of low-order sidebands accumulated. However the phases are incoherent, and thus the system is unable to demodulate. Therefore, the influences of high-order sidebands outputted from the modulators with different modulation depths are discussed in detail. From the analysis of high-order sidebands, one can know that when the radio-frequency signals satisfy the optimal phase formula, the ±1 st, ±3 rd, ±5 th, ±7 th order flat sidebands are generated by the first MZM, and the eightfold-frequency millimeter wave with a 4 th order sideband can thus be obtained through the second MZM without a filter. With this scheme, a pure eightfold-frequency millimeter wave can be obtained without large radio-frequency signals. For this scheme, not only the design structure is simple, but also the spectral purity is high.