Abstarct: Abstract Bent tubes are one of the most widely used products in various industries such as aerospace, military, oil, gas, and petrochemical industries, automobile manufacturing, vehicles, and home appliances. Tubes consisting of elbow joints and welded are not reliable for high technology products. Therefore, the production of integrated curved Tubes by forming method has been considered. Of course, the ovalization of the cross-section and the wrinkling of the tube's inner surface are unavoidable in the process of bending tubes. These defects lead to the inability to assemble the product, distorting the appearance and reducing the cross-sectional area inside the tube. Especially in cases where the fluid passes through the tube; It also causes problems such as fluid pressure drop. To overcome these defects, internal pressure is applied with the help of a fluid medium (water or oil), an elastic solid mandrel (polyurethane cylinder), or a granular medium such as sand or pellets. One of the important defects in the tube bending process is the spring back phenomenon, which is caused by the elastic behavior of the material after bending, and causes an unwanted change in the angle and radius of the bend. Recognizing the exact amount of spring back and its compensation in mold design, in addition to increasing the dimensional accuracy of the product, has a great role in reducing experimental trial and error and production costs. The spring back is dependent on the factors of the material such as yield stress, elastic modulus, strain hardness, and material anisotropy, as well as the process components such as the bending method, bend radius and angle, cross-section shape and dimensions, temperature and contact conditions. Analytical models presented to check the spring back of the tube are often done with the assumption of uniaxial or biaxial stress, and the effect of the radial stress of the tube is ignored in comparison with the circumferential and longitudinal stress. In the tube bending process using a mandrel to provide internal pressure, investigating the effect of using an intermediate medium providing pressure on the amount of spring back of the tube has not been considered so far. Therefore, in this thesis, a comprehensive analytical model for mextal tube bending under internal pressure is presented by considering three components of radial, circumferential, and longitudinal stress and using equilibrium equations and flow law related to the von-Mises yield criterion. Then, the return bending moment and spring back angle and the radius of curvature of the tube after loading are obtained. The effect of radial stress and bend geometry parameters, including curvature and bend angle, on longitudinal stress distribution, return moment, and spring back angle are investigated. In the experimental part, while designing and making a tube bending tool using a flexible mandrel; Tube bending is calculated by applying internal pressure for different values of the variables of the bending process and the spring back angle. The results obtained from the analytical solution will be verified with the experimental results. A numerical simulation of the tube bending process by applying the pressure of the medium will be used to measure the distribution and changes of the internal pressure.