Study and prediction of fatigue fracture is an important subject to design the machines and structural components which are influenced by vibration or cyclic loading. ‌‌Some cracks are produced during production process and the others can be created during the installation of components. Due to cyclic loads these cracks propagate to a crirical length and after that the result is component fracture. If an overload takes place during the cyclic loading so it will affect crack growth rate significantly. Using the fracture mechanic science is one of the methods of studing the fatigue crack growth. The fracture mechanics investigate the components load capacity so it is possible to study the life of componetns which are subjected to fatigue loads by using fracture mechanics. Gears are the most important machine components that are used to transfer power and motion. The fast growing industrial developments and economical considerations have led to the need for designing durable components with minimum weight. In order to achieve this goal, designers need to understand the failures mechanism of machine parts and do their calculations accordingly. Gears may fail with several different mechanisms. One of the most important is fatigue crack growth in tooth root. The tooth root is most exposed to crack initiation and damage because of high bending stresses. Since the gears are used in many machines and the gears tooth fracture lead to machines work stop, study and prediction of gears life can be so useful and efficient. The goal of this study is to estimate the main gearbox life of Hallden flying shear in cold rolling mill area of MSC. First of all kinematic analyse was done on Hallden flying shear and after that the maximum shearing force of cutting process was estimated. At the next step dynamic analyse was done by using the maximum shearing force. Dynamic modeling method was used the mass-spring modeling in ABAQUS to model the shafts and gears. The result of dynamic modeling was torques that was applied to gears and so the critical gear was specified. Then by using contact analysis the exact load distribution on critical gear teeth and the crack initial location were obtained. In order to obtain exact load distribution on the tooth face, conjugate action of the studied gear with its meshing gear has been analysed using contact elements. Finally the extended finite element method (XFEM) has been used to simulate 3D fatigue crack growth and obtain growth path. This method was introduced during recent years and modelling time has reduced considerably compared to previous works carried out on 3D crack growth in gears. To estimate the fatigue life of critical gear in main gearbox the Paris equation has been used. Paris law is used to calculate life cycles from the point when initial crack starts to grow until it reaches the critical length where fracture occurs. Also a code has been developed in ABAQUS environment taking advantage of this software’s capability of modeling static cracks by XFEM. The final results of analysis gave important and useful information about estimation of gears life and crack’s growth path and the final shape of the crack was found, so by using these information it is possible to prevent the sudden fracture of gear teeth in gearboxes of Hallden flying shear. Keywords: Fatigue, Gear, Fracture mechanic, Finite element method, Sheet metal shear