In this study, AISI 316 types of austenitic stainless steels were welded by FCAW (flux-cored arc welding) using E316LT1-1/4 flux-cored wire under various shielding gas mixtures containing CO2 at different ratios. Effects of mixed ratio of Ar and CO2 in the in the shielding gas on the microstructure, impact toughness, and microhardness of welded materials were studied. Stereo optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), and TEM/mapping analysis techniques were used for microstructural characterizations. The impact toughness values of the weldments were decreased as a result of the formation and growth of inclusions in the microstructure due to the increases amount of CO2 in the shielding gas. The hardness values and delta-ferrite amount in the weld metal were affected by depending on of the shielding gas mixtures.