In this study, the authors investigated the effects of residual chlorine on mortality, grazing and respiration of Labidocera euchaeta in laboratory. The grazing rate was evaluated by subtraction method of food concentration and respiration rate was measured using oxygen electrode. It was found that the lethal effect of residual chlorine on L. euchaeta increased with enhanced concentration and prolonged duration. The medium lethal concentration (LCs0) of chlorine for L. euchaeta in 24 h was about 0.58 mg/L and the safe concentration was about 0.21 mg/L. However, the grazing and respiration of L. euchaeta decreased by 32.6% and 18.9% when exposed to 0.2 mg/L residual chlorine for 4 h. It indicated that the physiological activities of zooplankton could be suppressed by the residual chlorine less than the safety concentration. Therefore, both survival and physiological activities of the organisms living in the thermal (nuclear) power plant discharging waters should be considered when carrying out the ecological risks assessment.