A photonic-assisted radio-frequency (RF) self-interference cancellation (SIC) scheme for in-band full-duplex radio-over-fiber system is proposed based on a dual-drive Mach–Zehnder modulator (DD-MZM) and a fiber Bragg grating (FBG). The received signal with an interference from the transmitter is applied to one arm of the DD-MZM, and the reference signal tapped from the transmitter is injected into another arm of the DD-MZM. By properly controlling the amplitude and phase of the reference signal, and adjusting the bias point of the DD-MZM, the RF interference can be cancelled in the optical domain. An FBG is used to convert the phase-modulated received signal to an intensity-modulated signal for information recovery. Since the self-interference is cancelled directly in the optical domain, the proposed SIC scheme is suitable for long-distance fiber transmission. A simulation is performed, where a cancellation depth of more than 50 dB is achieved. The proposed SIC system features good transmission capability and frequency tunability, which is only limited by the bandwidths of the DD-MZM and the FBG.