During the operations of the establishment of water or oil conveying pipes, the topography of the course necessitates that some percentage of the pipes need to be bent. During the bending, some defects such as ovality, wall thinning, wall flattening and wrinkling are made, and these defects limit the bending angle. To prevent the occurrence of such defects, the manufacturers of cold bending machines suggest the use of a mandrel which is settled inside the pipe in the bending zone, and it reduces the formation of geometrical imperfections to an acceptable extent. In Iran, installer companies refrain from using mandrel in bending of large diameter pipes. The absence of mandrel is more apparent in bending of spiral pipes, and the geometrical defects caused in them are more prominent. The main goal of this dissertation is the simulation of the cold bending process of large diameter straight and spiral pipes with the purpose of the prediction of geometrical imperfections in these pipes.In this research, the cold bending of straight and spirally welded pipes has been modeled in the ANSYS Software. The pipes used in this research have been of the Type X65 steel and in order to consider the Baushinger Effect, kinematic hardening model has been used. In the finite element simulation of pipe bending, the effect of parameters such as curve radius and Die length, pipe material, weld stiffness, diameter of the pipe and also anisotropy on geometrical defects has been studied. In addition, with the suggestion of a model for a mandrel simulation, the extent of producing an effect on geometrical defects has been explained. In the model suggested, the mandrel has been modeled with spring elements and its elastic modulus has been calculated in the way that its hardness be equivalent to that of the mandrel’s. Also, in this dissertation, process of spiral pipe manufacturing has been modeled and the residual stresses have been calculated.The results of this research show that increasing the bending radius causes an improvement in the quality of the bend. Besides, the consideration of the welding stiffness in the bending simulation of straight seam pipes causes a reduction in the ovality and wall thinning, but increasing the weld stiffness in spiral pipes contrary to straight seam pipes increase the ovality and intensifies geometrical imperfections in these pipes. Simulations show that sheet anisotropy pipe does not have a prominent effect on the extent of defects, while applying anisotropy on a spiral pipe has disturbed the symmetry of the residual stresses in the pipe wall. Using a mandrel inside the pipe has severely reduced the extent of ovality. By using a mandrel, wrinkling and flattening have been completely removed. Keywords: Spiral Pipe, Straight Seam Pipe, Residual Stress,Cold Bending,Mandrel, Geometrical Imperfection, Weld Stiffness, Plastic Anisotropy