A frequency-selectable and multi-channel wavelength division multiplexing radio-over-fiber system is studied. The system is able to transmit at most 4-channel frequency-selectable downstream millimeter wave (MMW) signals and 2-channel to 4-channel upstream signals. Simulation results show that the proposed system can generate MMW signals with different frequency multiplication factors by 7×8 digital optic switch, such as 2, 4, 6, 8, 10 and 12. After 50 km standard single mode fiber transmission, the power penalty of the downstream MMW signals is from 1.21 dB to 1.71 dB under the bit error rate of 10-9, while the power penalty for upstream signals is from 1.10 dB to 1.24 dB. The proposed system is successfully verified with characteristics of equalization and dispersion mitigation.