Abstract: Carbon isotopic reversal is a phenomenon often observed in the shale gas in some areas. Studies suggest that the reversal may come from various reasons, such as carbon isotope fractionation by diffusion and adsorption/desorption processes, mixing of primary cracking gas and secondary cracking gas and the reaction of shale gas with water and transition metals. In addition, high temperature, dissolution and tectonic activities may also contribute to some extent to carbon isotope reversal in shale gas. Carbon isotope reversal in shale gas is a significant indicator to prediction of shale gas yield zone and resource potential assessment, as well as fracturing effect and preservation conditions of shale gas reservoirs. However, there are still controversies. The dominant role of diffusion and adsorption/desorption remains uncertain; it is arguable that under what circumstances a mixture of primary cracking gas and secondary cracking gas may lead to carbon isotope reversal; and the reactants and products need to be identified in the reaction of shale gas with water and transition metals. Further studies are needed to settle these disputes.