Pressurized Water Reactor components are welded by Gas Tungsten Arc Welding (GTAW). To achieve good corrosion resistance and mechanical properties, Ni base alloy 690 is used to manufacture these components. The understanding of physical phenomena involved during welding and the prediction of induced residual stresses are crucial to guarantee high quality of these components. Welding induces drastic changes in the microstructure of the molten zone and heat-affected zone of metallic alloys especially for multi-pass welding. These changes may deteriorate the mechanical properties of the assembly. In order to reproduce the complex thermo-mechanical loading occurring within the heat affect zone, experiments on a thermo-mechanical simulator Gleeble 3500 have been carried out. In order to characterize the base alloy, isothermal tensile tests have been performed at various strain rates and temperatures (from 25 to 1100°C). A constitutive law has been proposed to predict the mechanical properties under different strain rates and temperatures. Tensile tests have also been performed after several thermal cycles to understand the effect of welding on mechanical properties of Ni alloy 690. In parallel, grain size evolution and carbide precipitation have been characterized and correlated to measured mechanical properties.