Chlorine inhibition on the denitrifing activity of activated sludge treating dairy wastewater was investigated in this study. Filamentous bulking was caused artificially by a sudden load of feed and monitored by measuring sludge volume index. In cases of the activated sludge and bulking sludge which were contacted with chlorine as 7.5 mgCl2/gVSS/day for bulking control, the decreases of specific denitrification of 32.2~40.4% and 43.5~46.5% were shown in comparison to the control group which was not reacted with chlorine, respectively. In continuous operation, it was observed that the removal efficiency of total nitrogen was more susceptible to chlorine than the removal efficiency of total phosphorus.