Abstract: The joining of dissimilar metals, T2 copper and Q235 mild steel plates with 4 mm thickness, was carried out using friction stir welding (FSW)technique.A defect-free weld with good surface appearance has been obtained.The results indicated that the resultant microstructure of the weld is associated with the thermo-mechanical effects during the FSW.In the near-shoulder zone at the steel side, the microstructure of the steel presents fine equiaxed ferrite grains which attributed to dynamic recry stallization due to the large strain and higher temperature, but in the heat-affected zone (HAZ)adjacent to the pin, the microstructure of the steel is composed of pre-eutectic ferrite(PF)and ferrite side plate(FSP), which is same as the original microstructure before welding because of the lower strain and temperature within this region.At the copper side, the grains in the HAZ present coarser due to the welding heat, but in the thermo-mechanical affected zone the grains become quite small due to the dynamic recrystallization induced by the welding heat and the strain.In the stir zone, the microstructure presents lamella composed of copper and steel plates alternately on the upper part, and vortex-like feature composed of copper, steel and the intermetallics from them alternately on the lower part, in which copper plates present fine equiaxed grains, and the steel plates present fine equiaxed grains or PF+FSP intercalation microstructures.