The authors report on the mineralogical features and carbon-oxygen isotope composition of the authigenic carbonates build-up of Samples TVG-1 and TVG-11 from Jiulong methane reef in the South China Sea. The results by XRD confirm that the samples are composed of miscellaneous carbonates, including aragonite, Mg-calcite and dolomite, while detrital minerals are less abundant. In Sample TVG-1, aragonite is replaced by calcite and the pseudomorph of the former is observed, suggesting that this sample was formed earlier than the other because phase transformation of aragonite to calcite has occurred. SEM observation reveals that micro-scale acicular aragonite is usually associated with bio-filaments which may be extra-cellular polymeric substances produced by microorganisms. These observations confirm that carbonates precipitation is related to heterotrophic microbial communities fueled by hydrocarbon seepage only having weak activity currently. Stable isotope composition of the carbonates is characterized by strong 13C depletion, with a ?13C value of –46.22‰ and –52.88 ‰ for TVG-1 and TVG-11, respectively, indicating that the carbon source is most probably related to anaerobic oxidation of methane. Another distinct feature of the carbonates is its enrichment of ?18O, +3.19‰ and +4.07‰ for TVG-1 and TVG-11, respectively. 13C depletion and 18O enrichment along with the micro-fabric textural evidence suggest that the Jiulong carbonates build-up might have been precipitated from the fluids originating from gas-hydrate decomposition, which indicates that this area may have gas hydrate resources.