The most critical issue in water distribution network design is determining the efficient diameter of a pipe. If the water transmission and distribution systems are optimized for a water supply project, significant cost savings can be realized. This can be accomplished by selecting the most cost-effective pipe size for the distribution and transmission systems. In this study, economical diameter of a pipe in a distribution system was determined by considering pipe flow constraints such as length, velocity and pressure head for branched network water distribution system. The most critical pipe flow characteristics were assessed within the distribution network's permitted range. The most cost-effective diameter of pipe in a distribution system is estimated using linear programming, with cost minimization as the objective function and length, velocity and pressure head as constraints. The suggested optimization tool's accuracy and efficiency were demonstrated using WaterGEMS10.01 to check the pressure, velocity and head loss gradients under steady state peak hour conditions in Mekane-birhane town branched water distribution network. For the sake of comparison, a distribution network with a total pipe length of 16.035 kilometers was developed utilizing three different alternative design approaches: conventional (trial and error), Darwin designer, and linear programming. According to linear programming, Darwin designer, and trial-and-error ordinary design cost optimization methods, the cost of the study area water distribution network was, respectively, 5.3, 5.5 and 6.4 million ETB. The result obtained from linear programming was compared to the result obtained from the trialand-error ordinary design and Darwin designer optimal value. Results indicated that linear programming was capable of obtaining a robust design with least cost. Therefore, linear programming design approach was the most cost effective. Key words: - linear programming, cost optimization, economical diameter, WaterGEMS, distribution system.