Design of a minimum cost canal section involves minimization of the sum of costs per unit length of the canal, subject to uniform flow condition. These costs are included with the costs related to covering every meter of channel length, the cost per cubic meter earthworks and costs associated with water loss (including seepage and evaporation losses) which is considered as the objective function in this study. Seepage and evaporation are forms of water loss in an irrigation canal, while the seepage loss depends on the canal geometry and the evaporation loss is proportional to the area of water free surface. In this study, the Manning’s Formula is considered as the main constraint equation. In addition, the minimum permissible velocity and Froud number are used for optimizing design canal as subsidiary constraints. The optimization algorithm based on directly search is used for optimizing the canal section. Presenting the results of optimization model in the form of the dimensionless curves, provides the possibility of easy design of optimized canal section in order to minimize costs per unit of length of canal. By analyzing the results of this study, it can be found that the trapezoidal canal with a given discharge have the maximum and minimum normal depth in side slopes of 0.75 and 1.5, respectively. Also, by increasing side slope in optimal design of triangular canals, the normal depth decreases and the width of water surface increases.