A new millimeter-wave radio over fiber (RoF) system based on optical frequency multiplication (OFM) technique is proposed, in which quadrature phase shift keying (QPSK) is implemented on optical wave in a parallel structure of Mach-Zehnder modulators (MZM). In the center station, light-wave injected into parallel DD-MZM is split into four optical carries, which are phase modulated by four microwave signals respectively, and then combined together. The light-wave is photodetected to generate an electrical signal consisting of harmonics of the microwave signal. The desired millimeter-wave signal is included in the harmonics, and can be maximized just by adjusting the phase modulation index. QPSK modulation is realized while millimeter waves are generated at the base station. This system overcomes the disadvantage of the traditional optical QPSK modulation in an mmRoF system. Simulation results verify the millimeter-wave generation technique, and the signal modulation and demodulation, showing feasibility of the proposed millimeterwave system.