Abstract: Carbon isotopic compositions of Sinian marine carbonates from the Yangtze Gorges sections near Yichang, South China, have been investigated. The δ13C values showed large variation during the Sinian. The marine carbonates have typical lowδ13C values of about -4‰ in the Nantuo Period. At the beginning of the Doushantuo Period the values increased rapidly, and reached +5～+6‰ in the early and middle time of the Doushantuo Period. The mass of 13Cvalues remained stably positive, ranging generally between +2.0‰ and +3.4‰. The large varations of carbon isotopic compositions of Sinian seawater are mainly due to violent growth and wide extending of acritarch and algae, rapid increasing of global sedimentary rate, and development of “oceanic anoxic event”, i.e. the existence of anoxic layer in deep seawater during that time.