Bacteria and picophytoplankton can be decomposed by chlorine in disinfection process of drinking water treatment, and organic matters released from them would contribute to assimilable organic carbon (AOC). Therefore, the specific AOC formation potential of each picoplankton, i.e. bacteria and picophytoplankton, in the chlorination were evaluated and compared in this research. The nutrient state of bacteria affects onto its specific AOC formation potential. The specific AOC formation potential of bacteria starved, which expected to be similar state to natural bacteria in a reservoir, was 0.8 × 10−13 g/cell. It was half of that of bacteria in the logarithmic growth phase. The difference of specific AOC formation potential of bacteria between in the logarithmic growth phase and in the starving condition would be caused by the amount of cellular organic carbons. The specific AOC formation potential of picophytoplankton was 6.4 × 10−13 g/cell, and 8 times higher than that of bacteria starved. The AOC formed by chlorination of picoplankton should be considered for AOC control in summer season when both bacteria and picophytoplankton multiply.