Abstarct: Before extraction of ore by open pit mining, it is necessary to determine the pit limit. Optimum open pit design is one of the most important designs in open pit mining. For estimation or assessment of this outline, economical block model is essential. This model can be achieved after exploration to ore body and creating of geological block model and applying costs and revenue of each block. A number of algorithms have been developed to determine the pit limits. These algorithms can be dividing into two groups, mathematical and heuristically methods. Heuristically methods have an advantage that can obtain solution(s) in a reasonable time. In some cases they can not produce true optimum pit limit. Mathematical methods are able to determine true optimum pit limit. but these algorithms are very complicated and need to considerable time to reach the solution(s). Hence open pit outline problem in this thesis is modeled by ant colony optimization algorithm. A 2-D model of blocks has been modeled and restrictions applied to it. At subsequent stages some techniques employed to solve the problem. A computer program developed (with Visual Basic 6® programming language) to solve the problem in a short time. With application of ant colony algorithm on problem, specified that this algorithm have a proper ability to determine the pit limit. Algorithm parameters are sensitive to dimensions of problem and with increasing of these dimensions, the answer can divergent soon. In each iteration, the best solution are kept for determination of fond optimum pit.