Abstarct: It less than two centuries past from the exploration of the first oil well in all of the world. Black gold has made an enormous development in the daily life of the human societies such that life without oil and its products will be unimaginable for the modern man. However, easy oil reserves are being spent thoroughly, and in most of the world’s reservoirs the production rate has obtained a downward trend. Therefore, the oil industry seeks to find new methods for increasing the production and yield of hydrocarbon reservoirs. Horizontal drilling technology is one of the few successful methods which has been welcomed increasingly. In horizontal drilling, contact between the well and the reserve is increased and production rate of the reservoir is also significantly increased. In contrast to the various advantages of this method, there are disadvantages such as heavy investment and the specific technology of drilling. Therefore, the success of drilling horizontal wells in reservoirs is of much importance. Having detailed knowledge about the reserves’ environment and being aware of the changes in different parameters of the reserves, decrease the probability of incidence of different problems in drilling horizontal wells and guarantee the success of this method. Mechanical Earth Model is a tool which can solve this problem by displaying the changes in different parameters of the reserves. In this thesis, the MEM has been used to determine the optimum direction for drilling horizontal well in Aghajari reservoir. The methodology includes at first the studies related to the literature from horizontal drilling to MEM. Then, the intended data have been collected from Aghajari reservoir and a 1D.MEM has been made. The 1D. MEM has been calibrated according to the results obtained from the experiments conducted on the cores. In the next step, the geological model of the reservoir has been made and gridding has been done. Then, a 3D. MEM has been made baxsed on the 1D. MEM and the geological models, and stress values have been estimated at the reservoir. In the last step, the optimum direction for drilling horizontal well has been determined. Using Mogi-Coulomb failure criterion, the suitable palace for drilling horizontal well in the reservoir has been determined.