Levels of organic matters in raw water from Huangpu River and treated water from different processes have been investigated. Dissolved organic carbon (DOC) and assimilable organic carbon (AOC) have been determined on ultrafiltrate (UF) (MW cut-off of 1K and 10K Daltons) samples. The organics in raw water are majority of lower MW (<1K Daltons) fractions. The conventional treatment processes are effective in removing higher MW (>10K Daltons) organics, while granular activated carbon (GAC) adsorption can reduce organics with lower MW significantly. The lower MW fractions correspond to 60%~70% of the AOC in raw and treated waters, and much higher than other MW fractions. This indicates that the AOC is mainly related to organics with lower MW. In order to decrease AOC level to achieve biostability in drinking water, the processes which can reduce lower MW organics effectively must be employed.