In order to determine the best operating rules for the Nakdong River, three cases were applied to analyze the simulated results of water supply capacity by HEC-ResSim model. This study discussed to present the best operating rules for conjunctive operating of existing the dams and new constructed the weirs through system network. The system network was constructed considering the water supply, the river environment and the operating facility. The water supply capacities are separately evaluated for each case applying the best rules. It is case1 that the dams are operated individually, case2 that the dams are operated in conjunction with the others dams, and case3 that dams and weirs are operated in conjunction with the others dams-weirs. Comparing the cases, case 3 has shown the best water supply capacity of the Nakdong River.