Water samples were collected in Changjiang estuary and then the colloidal organic carbon was isolated by mini tangential flow ultrafiltration(TFF) in situ.In the ultrafiltration process,the TFF system was washed and rinsed after every use,only with 200ml 0.1 mol/l NaOH,and 200ml Milli-Q Water,and Milli-Q water was used to be the blank test.In the blank test of the 500 KD ( 1 KD =1000 Daltons) and 5 KD ultrafiltration membrane,the concentration of dissolved organic carbon (DOC) in the permeate are 1.19mg/l and 1.14mg/l,the concentration of DOC in the retentate and ultrafiltrate are 1.19mg/l,1.14mg/l and 1.14mg/l,1.05 mg/l;Mass balance of DOC showed a good present recovery ( 102.22 9.64) ％ and ( 97.36 10.27 ) ％,respectively.In Changjiang estuary,surface water DOC concentrations ranged from 2.58mg/l to 3.23mg/l.The percentage of surface water colloidal organic carbon(COC) ranged from 0.52％ to 23.19％.In addition,COCs-500KD is 0.02-0.71mg/l,which is 60.75％-100.00％ of the total COC.The results indicate that the use of the mini TFF for size fractionation of natural dissolved organic matter in situ is reliable and efficient.