The use of high strength low alloy steels provides several potential advantages including lower weight, lower manufacturing costs, and ease of handling and transport. The progress in steel manufacturing technology has continually called for new developments in welding processes and consumables to produce weld metal deposits with mechanical properties essentially equivalent to the base metal. Controlling the weld metal microstructures as well as raising the welding productivity is critical factor for the development of weld metal of high strength steel to secure satisfactory mechanical properties and to reduce production costs. In order to meet the demand to apply 1000 MPa class steel to the fabrication of large scale steel structures, a weld wire for the 1000 MPa class steel has been under development to obtain the required strength and toughness, which depend primarily on the microstructure. In this work, the effects of shielding gas composition on the microstructure and properties of 1000 MPa grade deposited metals produced by metal active gas (MAG) welding have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO₂) (5%~30%), and the weld heat input was 13 kJ/cm. The properties of deposited metal with shielding gas of 80%Ar+20%CO₂ is the best, the yield strength is 980 MPa, meanwhile, its Charpy absorbed energy at room temperature and -40 ℃ are 72.6 and 52 J, respectively. The results show that the microstructure of the deposited metal, consisting primary of low carbon martensite and a few parallel bainite plate, became more interweaved bainitic packets as the CO₂ content of the shielding gas was increased. The initial bainite nucleated at austenite grain boundaries and subsequent bainite plates can form at the oxide inclusions of intragranular, which presented an intersected configuration and the microstructure was refined. The content of bainite palte and distribution morphology of martensite/bainite is the intrinsic reason attributed to mechanical properties of deposited metals. The content of bainite for deposited metal has an optimal proportion and more isn't necessarily better. It was also found that the area fraction, the size and the compositions of oxide inclusions in deposited metals were changed with increasing CO₂ content. The deposited metal as using 70%Ar+30%CO₂ has the minimum toughness because more large size oxide inclusions formed which are known to be harmful to the toughness.