Chlorination inhibition on the organic removal activity of activated sludge microorganism was investigated in this study. It is well known that chlorination improves the settleability of filamentous bulking sludge through the selective impediment of filamentous microorganisms. However, it is based on the declination of effluent water quality after actual chlorination in dairy wastewater treatment plant. In case of the activated sludge which was exposed in the suggested concentration of chlorine $7.5;mgCl_2/gVSS/day$ for the filamentous bulking control, decrease of organic uptake rate of $4.9~24.0\%$, and dentrification rate of $24.8~30.3\%$ ware shown in comparison to the control group which was not reacted with chlorine. As a result of comparing floc size of activated sludge microorganism, the average of floc diameter in the chlorine exposed group was $150;{mu}m$, which displays $25\%$ decrease compared with the control group.