A raw water sample was collected from Guangzhou section of Pearl River to support experiment studies. The sample were passed through Amicon YC-05, YM-1, YM-3, YM-10, YM-30, YM-100 and ZM-500 series membranes after being pretreated. The molecular weight distribution of the isolate for each membranes were analyzed, and the isolates were disinfected by chlorine and chlorine dioxide. The results showed that the RO isolate was the main dissolved organic matter (DOM) in the water sample, which proved that the water was highly contaminated by human activities, and it was the main THMs precursor in the disinfection process with chlorine and chlorine dioxide for all the isolates. Meanwhile, the disinfection process for water sample contain DOM that its molecular smaller than 500 were very different from raw water.