Abstract: Q550 high strength steel was welded by means of gas metal arc welding process without pre-heating.The effects of welding wires with different composition and strengths on the microstructure of weld metal and the mechanical properties of the joints were studied.Results indicate that acicular ferrite is the dominant microstructure in the weld metal produced by using MK·G60-1 welding wire.When the joint is subjected to tensile stress,cracks occur easily at pro-eutectoid ferrite presenting along the prior austenite grain boundaries.Both the tensile strength and impact toughness in weld metal and fusion zone are improved with the increasing of the content of acicular ferrite.The tensile strength of the joint made by MK·G60-1 wire is close to that of Q550 steel,and tensile samples fracture at the fusion zone.For both ER50-6 and MK·G60-1 weld metals,impact toughness is the best in the heat-affected zone and the worst in the fusion zone.Transgranular fracture with the mechanism of micro-void coalescence appears in fibrous zone in the fractured surfaces of impact specimens.It can be concluded from microstructure and fractographs that acicular ferrite corresponds to relatively large dimples while pro-eutectoid ferrite corresponds to the small ones.