Among the components of the hydroelectric plants, Penstock is highly valued. Penstock is a high-pressure pipe which directs water from dam reservoir or any high head water toward the turbines of the plant. Due to The high expenses of its production, installation and maintenance, it optimizing design is of paramount importance. Considering the optimizing methods, genetic algorithm has a special capacity for optimizing the engineering issues. In this paper, we have tried to optimize the dimension of penstock pipes through the genetic algorithm. The structure of genetic algorithm has also been studied using sensitivity analysis and the best case for optimization and reduction of weight of penstock pipes has been employed. As the results revealed, high combination probability and downward mutation create the best responses in optimizing the high-pressure pipes and this was able to reduce the penstock weight of Shahriar dam about 12.15%.