ABSTRACT: Carbon isotopes of inorganic carbon-bearing materials (gaseous CO2, dissolved inorganic carbon (DIC) and solid carbonates) sampled from four mud volcanoes in the northern Tianshan fold zone were investigated to study the mud volcanic carbon cycling process. The analysed carbon isotopes showed 13C enriched characteristics, the δ13C values of CO2, DIC and solid carbonates were in the range of 10.99–32.11‰, 17.02–36.22‰, and −7.28–12.40‰, respectively. The 13C enrichment of CO2 could be attributed to carbon isotope fractionation during the partial reduction of CO2 by methanogenic bacteria. The positive δ13C values in DIC may be caused by the carbon isotope fractionation during CO2-water-rock interactions. The mixture of carbonate precipitates from DIC in the water and carbonate minerals from the host rock resulted in a relatively less positive δ13C value in carbonates of the mud samples. The reduction of CO2 by methanogenic bacteria could be the primary mechanism for the 13C enrichment of inorganic carbon during the carbon cycling process in the mud volcanic systems in the northern Tianshan fold zone.