Surface water was collected from the Jiulong Estuary for determination of activity concentrations of uranium isotopes in different size fractions,namely,greater than 53,10~53,2~10,0.4~2 μm,10 000 u~0.4 μm and less than 10 000 u fractions by microfiltration and cross-flow ultrafiltration technologies. Results indicated that most of the dissolved uranium (<0.4 μm) existed in the low molecular mass fraction (<10 000 u),and the colloidal uranium-238 (10 000 u~0.4 μm) only contributed less than 1% of the dissolved uranium-238. The fractions of colloidal uranium in the dissolved phases decreased with the increasing salinity. A positive linear relationship between uranium-238 activities and salinities was observed for the dissolved,colloidal and low molecular mass fractions,indicating a conservative behavior of uranium in the Jiulong Estuary. In the particulate phases (>0.4 μm),the partitioning of uranium isotopes among different size fractions was controlled by the partitioning of particle concentrations. In the regions with salinities below 20,the partitioning of uranium-238 among different size fractions was as follows: 10~53 μm>2~10 μm>0.4~2 μm greater than above 53 μm. However,the order at the offshore station with salinities above 30 changed as follows: 0.4~2 μm>10~53 μm>2~10 μm greater than above 53 μm. The fraction of the 0.4~2 μm particles increased at the offshore station,suggesting the increased contribution of the authigenic uranium. The activity ratio of uranium-234 to uranium-238 in the dissolved phases,including the low molecular mass fraction and the colloidal fraction,was larger than unity,showing the occurrence of excess uranium-234. In contrast,the activity ratio of uranium-234 to uranium-238 in all size fractions of the particulate phase was close to the equilibrium value (1.0). The observed different values of the activity ratio of uranium-234 to uranium-238 in the dissolved phase and the particulate phase were ascribed to the preferential leaching of uranium-234 and the small amount of the leaching particulate uranium. The mass ratio of thorium-232 to uranium-238 also showed different values between the dissolved phase and the particulate phase. Mass ratio of thorium-232 to uranium-238 in the dissolved,colloidal and low molecular mass fractions was less than unity,while those in the different size fractions of particulate phases were larger than unity,reflecting a different behavior between uranium and thorium during their transport into the ocean. Surface water was collected from the Jiulong Estuary for determination of activity concentrations of uranium isotopes in different size fractions, namely, greater than 53, 10-53, 2-10, 0.4-2, 10 000-0 0.4, and less than 10 000 u fractions by microfiltration and cross-flow ultrafiltration technologies. Results indicated that most of the dissolved uranium (<0.4 μm) existed in the low molecular mass fractions (<10 000 u), and the colloidal uranium- 238 (10 000 u ~ 0.4 μm) only contributed less than 1% of the dissolved uranium-238. The fractions of colloidal uranium in the dissolved phases decreased with the increasing salinity. A positive linear relationship between uranium-238 activities and salinities was observed for the dissolved, colloidal and low molecular mass fractions, indicating a conservative behavior of uranium in the Jiulong Estuary. In the particulate phases (> 0.4 μm), the partitioning of uranium isotopes among different size fractions was controlled by the partitioning of parti cle concentrations. In the regions with salinities below 20, the partitioning of uranium-238 among different size fractions was as follows: 10-53 μm> 2-10 μm> 0.4-2 μm greater than 53 μm. However, the order at the fraction of the 0.4 ~ 2 μm particles increased at the offshore station, suggesting the increased contribution of the authigenic uranium. The activity ratio of uranium-234 to uranium-238 in the dissolved phases, including the low molecular mass fraction and the colloidal fraction, was larger than unity, showing the occurrence of excess uranium-234. In contrast, the activity ratio of uranium-234 to uranium-238 in all size fractions of the particulate phase was close to the equilibrium value (1.0). The observed different values ​​of the activity ratio of uranium-234 to uranium-238 in the dissolved phase and the particulate phase were ascribed to the preferentia l leaching ofuranium-234 and the small amount of the leaching particulate uranium. The mass ratio of thorium-232 to uranium-238 also showed different values ​​between the dissolved phase and the particulate phase. Mass ratio of thorium-232 to uranium-238 in the dissolved , colloidal and low molecular mass fractions was less than unity, while those in different size fractions of particulate phases were larger than unity, reflecting a different behavior between uranium and thorium during their transport into the ocean.