In recent years, high and ultra-high strength steels have been developed and used in light-weight constructions such as the structural members of mobile equipment in order to reduce weight and fabrication costs and to enhance the performance. Welding of steels with yield strength of more than 900 MPa is particularly challenging because of the toughness requirements for the weld metal, which calls for welding consumables of high strength and good toughness. Weld metals have been produced for a variety of welding methods with yield strength up to or above 1000 MPa, but their impact toughness remained only at medium yield strengths. Proper microstructure is the key to meeting this requirement, and its final microstructure depends on the chemical composition and cooling rate. For the deposited metal produced by gas metal arc welding (GMAW), the composition is dependent on the welding wire and shielding gas. The cooling rate of the weld metal is controlled by a combination of heat input and heat extraction. It is known that the addition of CO₂ to argon based shielding gas is effective for improvement of productivity in GMAW welding of steel. Through the chemical reaction in the welding arc, CO₂ in the shielding gas can affect the chemical composition of the weld metal, and its microstructure. The 1000 MPa grade deposited metals was welded with GMAW, and the effects of shielding gas composition (Ar+(5%~30%)CO₂, volume fraction) on the general compositional and microstructural characteristics of deposited metals, including nonmetallic inclusions, were experimentally characterized with SEM, EBSD and TEM. The microstructure of the deposited metals is mainly composed of martensite and bainite. With the increase of CO₂ content (5%~30%), the strength of the deposited metals decrease slightly and the impact toughness increases first and then decreases. Meanwhile, the transformation range (B₅₀-M_s) of the deposited metal increases, the bainite content increases (8%~29.6%) with the quantity of inclusions that are suitable for bainite nucleation increases, and the nucleation position changes from the original austenite grain boundary to the common nucleation on the original austenite grain boundary and inclusions within the grain. At the same time, the microstructure morphology of the deposited metal changes from parallel to intertexture, which presented an intersected configuration and microstructure refinement.