The generation of optical millimeter waves via the improved phase modulator in a RoF system and the transmission character of the signal are theoretically investigated.A new phase modulating scheme is proposed,in which the sidebands are separated by wave length demultiplexer and one of them doesn’t feed digital signals,thereby the phase wake-off is restrained. The generation of optical millimeter waves via the improved phase modulator in a RoF system and the transmission character of the signal are theoretically investigated. A new phase modulating scheme is proposed, in which the sidebands are separated by wave length demultiplexer and one of them doesn ’ t feed digital signals, thereby the phase wake-off is restrained.