Three cost functions of wastewater treatment plants in industrial districts were analyzed and discussed. From the results, the effluent charge rate can be proposed in order to internalize parts of the external cost of wastewater pollution. The charge base of the effluent charge shall be the total pollutant amounts in the effluent. The charge rate shall be set up according to the results of cost functions for various treatment efficiencies. The marginal cost of NT$790 (US$30.4) per noxiousness unit (F=1,000 m3/day, Q=5,000 noxiousness units/year, treatment efficiency = 90%) was recommended to be the initial charge rate. This cost should be updated regularly and the charge rate should be raised according annually.