Selection of the optimal tolerance for the components is one of the most important and complex issues in the design of the parts and products especially in sensitive and accurate industries (like as aerospace). Generally the design engineers tend to consider close tolerances in terms of product performances and high reliability, on the contrary to manufacturing engineers that used to open tolerances because of easier, cheaper and practicality of manufacturing processes. Study the balance between tolerance and other factors affecting it, such as quality, cost and production, is the most important issue in this case. In this regard, there are numerous articles and methods. In this paper we use the tools of statistics and mathematics as" response surface method", first method of diagnosis of the critical tolerances describes and then tolerance of the optimal allocation - in case the cost of production or quality characteristic is optimized- has been examined