Metal bellows or expansion joints are corrugated tubes which are used in connection points of piping systems to compensate movements, vibrations or even misalignments of the connected parts. Tube hydroforming, roll-forming and welding the individual rings are the main processes by which metal bellows are manufactured. Among different manufacturing methods, tube hydroforming is widely to produce the metal bellows in a broad range of sizes. There are two main steps in hydroforming of metal bellows. These are commonly known as tube bulging and folding. In tube bulging step the internal pressure in gradually increased to a maximum allowable value. Then in folding step, the die is gradually closed while the pressure is kept constant. The maximum allowable pressure which can be achieved in tube bulging step is a key parameter in designing the hydroforming of a metal bellows. This pressure is mainly based on designer experience and is usually determined via a try-and-error procedure. In this thesis, a semi-analytical approach is presented to study the tube deformation during bulging step, in hydroforming of a meal bellows. For this purpose, the tube wall curve is approximated by a circular arc, during deformation in bulging step. The material is assumed to be isotropic with a rigid-linearly hardening stress-strain curve in simple tension. Assuming the process to be axially symmetric, the equilibrium equations are derived for a proper element of tube surface. The equilibrium equations are then integrated to determine stress distribution in the tube wall. Some finite element simulations have been carried out to show the validity of the computed stresses. The integration constant should be determined in terms of the boundary conditions. However it is explained why this approach cannot be applied due to different pattern of deformation during the initial stages of bulging. Therefor an alternative approach is presented based on the ratio of stress components. On the other hand, the strain components are determined based on assumed deformation geometry. Hence the equivalent strain can be computed in terms of maximum bulge. The maximum bulge can be regarded as the main geometrical parameter by which the deformation during bulging can be characterized. The relationship between the equivalent stress and equivalent strain will then give an equation for pressure-maximum bulge. Since no analytical study was reported before, again finite element simulations were employed to investigate the validity of presented approach. A very close agreement was observed. This evidently shows that the results of the presented model are accurate enough to be used to predict tube deformation during bulging step, in hydroforming of a metal bellows. Keywords: Bellows, Hydroforming, Tube Bulging