The isotope ratio of volatiles is a powerful tool for investigating the origin of solar system materials and for revealing the possible chemical link between the solar system and the interstellar medium. ALMA observations provide carbon isotope ratios in the protoplanetary disks, for example, carbon fractionation is occurred in CO and HCN in the disk around TW Hya, which could be induced by the isotope exchange reactions when the elemental carbon is more abundant than the elemental oxygen (C/O> 1 in gas-phase). In this work, we present effects of gas-phase C/O ratio on the carbon isotope ratio in the disk using the chemical models.