The effects of welding conditions of gas metal arc welding on the elements loss of solid wire, oxygen content and impact toughness of weld metals were studied. Deoxidizing elements loss was increased with increase of arc voltage in both short-circuit transfer mode and globular transfer mode. It is believed that increase of arc voltage results in increase of reaction time between elements in the droplet and surrounding gas at the end of wire and in the arc column. Based on the thermodynamic equilibrium model, the oxygen content of weld metal can be predicted with the content of silicon and manganese as following : [%O] = $K([%Si][%Mn])^{-0.25}$, K = -15518/T+6.01. The equilibrium temperature was dependent on shielding gas, and it was 187$0^{circ}C$ for $CO_2$ gas and 180$0^{circ}C$ for 20%$CO_2$-80%Ar gas. The oxygen content of weld metal which shows maximum impact toughness was varied with deoxidizing alloy system of wires, 0.041 wt% for Si-Mn type wire and 0.026 wt% for Si-Mn-Ti type wire.