The aim of the application of high strength steel to mammoth structures is to maximize their allowable stress in designing and at the same time reduce their weight as a whole by the use of high strength steel in critical parts of the structures. In accordance with the existing concepts of elastic design, high strength steel has been generally used for the members subject to higher stress or inducing a stress concentration in the structures built up by welding together high strength steel and mild steel. However, the strain behaviors should be considered as one of the most important factors in the application of high strength steel to the structures, because the high strength steel is more sensitive to the strain concentration than mild steel is. In the previous report, the plastic deformation behaviors of the smaller size of composite weldment, in which high strength steel and mild steel are symmetrically arranged about the longitudinal axis, have been investigated under static tension parallel to the weld line. There are still problems of the similarity of the strain behaviors of the larger size of composite weldment to those of the smaller one. In this research, mechanical behaviors of the wide specimens having the heterogeneity are investigated in various combinations of the size of dissimilar steels with different strength. It is concluded as a result that ultimate tensile strength and yield strength of the wide specimens of composite weldment can be estimated by the strength of each steel and the hybrid ratio of high strength steel, and that their equivalent uniform elongation increases with the rate of mild steel to high strength steel.