Introduction: Water quality protection along rivers involves water quality monitoring and assessment, establishing water quality goals, and controlling pollutant discharges, so that an acceptable level of water quality is maintained.The control of water quality at various locations along a river requires determining the optimal pollutant removal levels at point and nonpoint pollution sources that would yield a satisfactory water quality response in a cost-effective, equitable, and efficient manner (Burn and McBean 1985; Burn and Yulianti 2001). This is known as the “Optimal Waste Load Allocation.” Typical multi objective optimal waste load allocation problems address minimization of the total treatment cost and minimization of the inequity among the pollutant dischargers, subject to constraints on satisfaction of a specified dissolved oxygen (DO) standard at all the check points along the river (Burn and Yulianti 2001). In this research, the capability of Multi objective Ant Colony Optimization algorithm is applied to a multi objective waste load allocation problem.