This section examines the microstructure and mechanical properties obtained from the joining of S355JR and S700MC structural steels, widely used in industry, by the GMAW method. While the effects of different shielding gases were not clearly observed in microstructure examinations performed with an optical microscope, differences were detected in the weld metal and heat-affected zones (HAZ) according to the properties of the base metal. In general, grain growth in the HAZ and weld metal, as well as a dendritic structure in the weld metal, were observed in all sample steels. In addition to the differences in metal structures, the shielding gas was one of the determining parameters in the hardness values   of the materials. The fracture surfaces formed as a result of tensile tests were examined depending on the grain growth, hardness values, and mechanical properties of the base metals.