The isotopic composition of dissolved inorganic carbon (DIC) in marine and estuarine environments has been studied in some detail because of its role in determining the isotopic composition of inorganic/organic matter and its applications to the study of various natural processes. The δ13CDIC values of sea water are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. In this paper, based on the description of the stable isotope geochemical characteristics of dissolved inorganic carbon in northern South China Sea, the following conclusions were drawn: 1) the δ13CDIC values of natural waters are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. And the low seawater δ13CDIC values of northern South China Sea are accompanied by a low level of sea primary productivity in this region; 2) according to the parameter relationship (δ13CDIC-1.1 PO4) between δ13CDIC and PO4, it is indicated that northern South China Sea is the source of atmospheric CO2; and 3) nitrogen (N) and phosphorus (P) are the elements which are the basis for phytoplankton growth. Based on the analysis of seawater δ13CDIC values and PO4 and NO3+NO2 concentrations, our data show that N is a limiting nutrient in northern South China Sea. The isotopic composition of dissolved inorganic carbon (DIC) in marine and estuarine environments has been studied in some detail because of its role in determining the isotopic composition of inorganic / organic matter and its applications to the study of various natural processes. The δ13CDIC values ​​of sea ​​water are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. In this paper, based on the description of the stable isotope geochemical characteristics of dissolved inorganic carbon in northern South China Sea, the the following conclusions were drawn: 1) the δ13CDIC values ​​of natural waters are controlled by the sources and sinks of carbon and the results from isotope fractionation among solid, dissolved and gaseous phases. And the low seawater δ13CDIC values ​​of northern South China Sea are accompanied by a low level of sea primary productivity in this region; 2) according to the parameter relationship ( 13CDIC-1.1P04) between δ13CDIC and PO4, it is indicated that northern South China Sea is the source of atmospheric CO2; and 3) nitrogen (N) and phosphorus (P) are the elements which are the basis for phytoplankton growth. Based on the analysis of seawater δ13 CDIC values ​​and PO4 and NO3 + NO2 concentrations, our data show that N is a limiting nutrient in northern South China Sea.