For researching the spatio-temporal variation of the stable isotopic composition of the riverine dissolved inorganic carbon (DIC), we had carried out a survey throughout the hydrologic year during which the δ¹³CDIC of the surface water and its physicochemical pa-rameter were examined along the Xijiang River Inner Estuarine waterway from September 2006 to June 2007. There was a striking seasonal variation on the average δ¹³CDIC, as the average δ¹³CDIC in summer (?13.91‰) or autumn (?13.09‰) was much less than those in spring (?11.71‰) or winter (?12.26‰). The riverine δ¹³CDIC was controlled by decomposed condition of the riverine organic matter linking the seasonal variation of the physicochemical parameter in the surface water according to the correlation analysis which indicated notable relations between δ¹³CDIC and water temperature (p = 0.000; r = ?0.569) or between δ¹³CDIC and oxide-reduction potential (p = 0.000; r = 0.646). The striking positive correlation between δ¹³CDIC and the sampling distance happened in the summer rainy season, while striking negative correlation happened in the spring dry season, indicating that river-sea interaction influenced water physicochemical parameters and controlled the riverine DIC property in the survey waterway. In view of the riverine δ¹³CDIC decreasing for the decomposition of the ter-restrial organic matter in the rainy season in summer and increasing for the briny invaded zone extending in the spring dry season along the waterway from the Makou gauging station to the Modaomen outlet, the δ¹³CDIC spatio-temporal variation was closely related to the geographical environment of the Xijiang drainage basin.