Small-scale tension specimens taken transversely from similar weldments of 12% Cr steel as well as small- and large-scale specimens from dissimilar weldments of 12% Cr/1% CrMoV with 12% Cr deposit were tested to obtain creep rupture data at 550°C. For the similar weld, the rupture time of the specimens was considerably longer than the rupture time determined in uniaxial creep tests of the weakest heat-affected zone (HAZ) microstructure. Results of finite element analyses have shown that this is caused by a constraint imposed on the weakest HAZ by the surrounding microstructures with higher creep strength. For the dissimilar weld, the creep rupture strength of large-scale specimens is higher than that of small-scale specimens. Finite element analyses have shown differences in the stress states developing in small- and large-scale specimens, which lead to different rupture times.