We investigated how various water treatment processes affect AOC (assimilable organic carbon) and MAP (microbially available phosphorus), which nutrient limits microbial growth between organic carbon and phosphorus, and what extent AOC is related to the MW fractions of NOM for raw waters of the Han River and process waters of two water treatment plants in Korea. In view of our results so far achieved, a significant difference was observed between MAP and MAP-P (potential MAP), while little difference was observed between AOC and AOC-P (potential AOC) for raw and pre-chlorinated waters, suggesting that organic carbon limits microbial growth instead of phosphorus. Conventional treatment processes poorly removed AOC-P (42.8%), since the AOC, which was mainly related to the small-molecular DOC, was hardly removed by conventional processes. Oxidants such as chlorine and ozone increased both of the AOC (AOC-P) and MAP (MAP-P), particularly ozone having the stronger oxidation power resulted in a larger increase. However, BAC and GAC removed AOC to less than 60 μg/L which meets the level for biological stability.