Due to the simplicity, low costs and reliability the welding process is one of the frequently used engineering methods of joining structural components in many industry fields. The large localized heat generation during welding leads to residual stresses and deformations. Tensional residual stress along in combination with additional stress due to exploitation reduces the durability of the construction, while plastic deformations cause inconveniences during the structure assembly. Removal of residual stress and deformation, using conventional post-weld thermal or mechanical treatments, represents an additional cost and prolongs the production time. Therefore, striving to reach a solution in construction phase, which will reduce the residual stress and deformation, different numerical methods are being used. In this paper a numerical simulation of a welding process of a T-joint fillet weld was carried out. To solve the problem of the heat transfer combined with nonlinear mechanical analysis the sequentially coupled method was used. The paper consists of nine chapters. Chapter 1 is a introduction with the defined welding process problem. In chapter 2 the theory of the residual stress, deformation and the behaviour of elastoplastic material was explained which follows to chapter 3, where are given the basics of thermodynamics of the welding process. Furthermore, the chapter 4 shows the heat-mechanical analysis after which the application of the finite element method on the simulation of the welding process is described. In the same chapter the finite elements used in the heat and mechanical analysis are described. Also, the same chapter gives the explanation of the verification proces by comparing the numerical results of the reference model (M1) on the example of T-joint panels with available soultions from the literature. The chapter 6 deals with the impact of thermal boundary conditions on deformations (deflections) and the longitudinal residual stress of the horizontal T-shaped panel. In this paper the cooling of welding is through natural convection on the environment temperature. The reference model M1 with defined convective heat transition coefficient is compared to the M2 model where the convective heat transition coefficient is defined as depending on temperature. Within this chapter it is concluded that the way of modeling of the convective heat transition coefficient does not have a big influence on the distribution of the temperature, deflection and residual stress. Chapter 7 observes the influence of mechanical boundary conditions on deflection and longitudinal residual stress for model M3 in which the encastre is present during the whole simulation and the models M4 and M5 where the mechanical boundary condition, encastre, is excluded after the model is cooled on the environment temperature (model M4) or right after welding (model M5). It is concluded that the deflections of free welded model M1 are much bigger than in case of the models that are encastred only during the welding process. Also, a bigger residual stress is noticed in permanently encastred model (M3) in comparison with the other three models. In chapter 8 the construction was preheated on a previously defined temperature. The impact of the temperature of preheating is expressed on deflections of the T-joint horizontal plate and on the longitudinal residual stress. The reference model M1 is compared with models M6 and M7. The preheating temperatures were 100°C (M6) and 150°C (M7) and it is showed how the preheating has an positive influence on reduction of the mentioned results.