An optical single sideband modulation radio over fiber (RoF) system with tunable carrier-to-sideband ratio (CSR) based on long period fiber grating (LPFG) is presented. The system mainly consists of five polarization controllers (PC), a polarization beam splitter (PBS), two parallelized long period fiber gratings (LPFG), and a polarization beam combiner (PBC). By tuning the polarization controller placed in front of the polarization beam splitter, continuously tunable carrier-to-sideband ratio can be obtained. The impacts of modulation index, transmission peak value of LPFG, and extinction ratio of the Mach-Zehnder modulator on CSR are discussed. Simulated results for transmission over 40-km single mode fiber (SMF) show that the receiving sensitivity increases by 5.7 dB when CSR changes from 13.22 dB to 5.4 dB which matches well with theoretical analysis.