Abstract: Tensile stress and strain field evolutions of metal butt weldments of 15 groups with different mechanical property parameters were analyzed by elastic-plastic finite element method.Results showed that there was a knee point on nominal stressdisplacement curve for all tested butt weldments and at the knee point yielding across the whole thickness section occured, which was confirmed by tensile test of undermatched butt weldments; that based on the analytical and test results, nominal yield strength was defined as the ratio of nominal yield force to initial section area of base metal.