The study the relationships among the element redistributions, the microstructure at various positions across the weldments, and local mechanical properties represents the basic source for advanced design of stable welds of dissimilar alloys. The kinetics of phase transformations in diffusion-affected zone determine weld stability in many cases. The processes in welded joint of progressive steel P91 (8,5wt% Cr -0,88%Mo-0,23%V-0,045%N-0,1%C) and 3Cr1Mo electrode (2,73wt%Cr-0,96%Mo-0,12%C) were simulated using combination of CALPHAD method(1) and the same method as can be found in DICTRA programme(2, 3, 4) . The thermodynamic database STEEL11.TDB(5) and mobility database DIF12.TDB(6) were applied for the simulations of the weldments at long treatment . This two databases were developed in Brno and they are assessed for calculation of phase diagrams and the simulations of the weldments at temperatures about 500oC too. The equilibrium calculations using CALPHAD method allowed us to predict phase diagrams for both 3Cr1Mo and P91 steels. The diffusion and phase transformation processes occurred in 3Cr1Mo/P91 welded join were simulated using DICTRA programme. It enabled us to evaluate time-distance dependences for phase compositions and element redistribution in diffusion-affected zone at temperature range 700-575oC. This simulation results at different temperatures are complemented with experimental results. The most interesting results concern carbonitride, and high molybdenium carbide phase transformations. The described method can be used for weldments design.