The building foundations are responsible for transferring loads to soil or bedrock that are made by superstructures. The stresses transmitted to the soil layer under the foundation must be in the limit that the shear failure doesn't happened and also the settlement of soil does not exceed the allowable limit. Because of the design mistakes, foundation deterioration, corrosion foundations due to sulfate attack and other environmental factors, the foundation might lose its bearing capacity, then leads to building collapse. Demolition of structures and constructing a new building is not efficient. Especially in places like historic buildings, this would be impossible. There are a lot of approaches and methods for strengthening the foundation. One of the most famous and successful methods which have been used in last decade is using micropiles due to some reasons like, ease of construction, reducing the construction costs, appropriate performance in sites with limited access, slightly changing in soil, sufficient bearing capacity for strengthening existing structures. In many of strengthening methods, foundation or soil is changed and foundation is getting strengthened by more rigid elements like piles or micropiles. Using of such methods is not economic and on the other hand, for vulnerable structures or historic buildings it would be impossible and also could be risky. So, the best strengthening methods are which ones that do not change soil. One of the newest method is the strengthening of the shallow foundation by non-connected micropiles. According to the last researches and studies, the bearing capacity can be increased up to four times. In this thesis, the strengthening of the square, circle and rectangle shallow foundations with concrete slab and pre-tensioned micropiles have been researched. For this purpose, our approach is based on numerical modeling by using FLAC3D. In first place, some microplies with specified number, distance, length and arrangement are placed around the foundation. After that, the concrete slab is placed on these microplies. micropiles’s cable is getting placed in the concrete slab and tensioned. The reaction of these pre-tensioning forces causes put pressure of the concrete slab on soil and playing the role of surcharge load for the foundation.In this method, the function of these micropiles of bearing capacity improvement Has been studied. Then the effect of length, distance, diameter, and the numbers of the micropiles on the bearing capacity, has been researched. After that, the concrete slab will be placed on the micropiles and the function of micropiles and concrete slab are examined. In the next step, by creating pre-tensioned in micropiles, different values of this parameter has been researched. Then the width of the concrete slab getting changed and the effection of them and pre-tensioning are studied at the same time. In this thesis, the allowable bearing capacity, ultimate bearing capacity and the settlement of shallow foundation in different cases and situations have been compared. The results have shown that strengthening by the concrete slab and pre-tensioned micropiles, have different effect on the soil, according to its foundation and its type. With this method, just the ultimate bearing capacity will increase and the allowable bearing capacity doesn’t Increase much more because of the soil settlement. Key Words: strengtheningthefoundation , micropiles, p re-tensioned micropiles, Bearing capacity improvement