Abstract: Based on the cross flow ultrafiltration technique, dissolved organic carbon and inorganic nitrogen of the colloidal phase (1 kDa-0.45 μm), solution phase (<1 kDa) and “dissolved phase” (<0.45 μm) in the natural water of Jiulongjiang Estuary were separated and extracted. The water environmental factors impacting on their physical and chemical properties were analyzed preliminarily and their origin and transformation were further discussed. The results show that the membrane blank and the mass balance of cross flow ultrafiltration process could meet the technical requirements. The dominant status of dissolved organic carbon, nitrite nitrogen, ammonia nitrogen, nitrate nitrogen and inorganic nitrogen were truly dissolved phase (<1 kDa), and their concentrations in the colloidal phase were in range of 0.207-0.810 mg/dm³, 0.001-1.870 μg/dm³, ND-2.08 μg/dm³, 0.62-79.30 μg/dm³ and 1.07-81.10 μg/dm³, respectively. Terrestrial inputs were important factors affecting the COC in Jiulongjiang Estuary.