The influence of the modulation index of the radio frequency (RF) signal on the optical carrier on the relation between power of the carrier component and that of the sideband component of the optical millimeter-wave signal generated by single sideband (SSB) modulation in the radio over fiber (ROF) system is theoretically and numerically investigated. In order to address the problem that when the carrier component is much larger than the sideband component in the optical SSB millimeter-wave signal that the receiver sensitivity of the radio over fiber (ROF) system is relatively low,the determinant of the power of carrier component and sideband component is analyzed. It is shown that an optimum RF modulation index can bring on a balance between the powers of these two components. It is verified by simulation results that the optimum RF modulation index can make the power of sideband equivalent to that of carrier component,thus results in the best receiver sensitivity of the radio over fiber system.