Ozone-assisted photocatalytic oxidation was employed for the degradation of gas-phase toluene in this study. TiO2/graphene (GR) composites with different addition ratios (0.1, 0.5, 1 and 2wt%) of graphene were synthesized. XRD, SEM (with EDS), TEM, BET, UV-vis DRS, PL and photochemical measurements were conducted to investigate the physical, optical and photochemical properties. The results indicated that TiO2 nanoparticles were smoothly loaded on the graphene nanosheets, and the as-synthesized TiO2/GR composites possessed excellent properties such as extended light absorption range, efficient charge separation and transfer ability, and broadened lifetimes of photo-generated carriers. The effect of water vapor in the ozone-assisted photocatalytic oxidation reactions was also studied, which was found to be essential for the degradation of toluene. When the relative humidity was 80%, TiO2/GR (0.1wt%) exhibited the highest toluene removal efficiency of 97.2% among all the samples, and other TiO2/GR composites also exhibited better toluene degradation performance than bare TiO2. Furthermore, the introduction of ozone was helpful for the improvement of photocatalytic performance. The promotion mechanism was proposed that the addition of an appropriate amount of graphene in TiO2 was helpful for the separation and transfer of photo-excited carriers, thereby reducing the recombination of e(-)/h(+) pairs and promoting the formation of oxidizing species such as oxygen negative radicals (O-3(-), O-) and hydroxyl radicals (OH). Overall, this work could provide a new insight into the fabrication of TiO2/GR composites as high performance photocatalysts in the environmental protection issues.