now days, natural gas plays an ompormatant role as a source of clean energy. the addition of gas pipelines. in the regrad, the new design and construction process by considering the technial specification is a must. using the obtained optinum gas pipeline's diameter, the investment cost and gas opartions can be minimized. gas distribution pipeline network consisits of nodes that respresent points of consumers and suppliers connected by a pipe. assuming the gas flow in a steady state, pipe networks are modeled into a nonlinear equation system from gas flow equations in the pipe. this model system solved by genetic algorithm to obtain the optimun gas pipeline's diameter with an investment cost of the pipe system as an objective function and specification of pressure on a node as a constraint. the optimization process is optimization of pipe specifications which available on the market (ANSI/ ASME ) eith 64 kind of diameter of range from 3 to 16 inch. at the end of the paper, a case study the optimization of gas pipe diameter in the region X is presented. from these case studies can be concluded that the genetic algorithm can determine the optimun pipe diameter which gibes the lowest investment costs while still consistent to the techical specifications that have been determined.