The effect of piece-wise welding on thermal and mechanical response of thick plate weldments is studied by the use of a 3-D thermo-viscoplastic model. Anand’s viscoplastic model is used to simulate the rate dependent plastic deformation of welded materials. Modeling of the welded region in the present study has been done by the use of the “isothermal melting pool” approach. The temperature dependency of thermal and mechanical properties of material is considered in analysis and the effect of welder characteristics such as welding speed and welding lag between sequences are introduced into the model. In addition, the element Rebirth technique (ERT) is incorporated into the model to introduce the effect of material of welds that added to the weldments during the welding. The model was compared with the results of two analytical and experimental works. The results of the present model are feasible and clearly show the relation between the order of joining blocks and distribution of thermal loads. They also demonstrate the role of this kind of welding sequence on the formation of residual stresses and distortion in thick plate weldments.