A novel scheme for linearity improvement of optical modulator is proposed for radio over fiber (ROF) system. This modulator is named ring and phase assisted Mach-Zehnder modulator (RPAMZM). A microring and a phase modulator are coupled to each arm of a normal Mach-Zehnder modulator respectively in order to compensate its sublinear characteristics transfer. Analyses under loss-less condition, loss condition and spur-free dynamic range (SFDR) are proposed. It is found that a high linearity can be achieved by properly setting the coupling coefficient under lossless condition. A corresponding SFDR of 19.68dB is achieved. When loss is considered, the main nonlinear terms, including the 2nd and the 3rd term, can be eliminated by properly setting the coupling coefficient and bias point. A corresponding SFDR of 15.74 dB can be achieved.