Abstract: Dissimilar T2 copper/316L stainless steel joints without defects were prepared by gas tungsten arc welding with Cu filler wire. Microstructure evolution and mechanical properties the joints were investigated. When Cu filler wire was used, the microstructure of copper/steel weld was largely affected by liquid phase separation between iron and copper. Fe-rich spherulites produced by primary liquid phase separation distributed in Cu-based weld. Inside the Fe-rich spherulites, minority Cu-rich spheres formed by secondary liquid phase separation. During solidification, the internal precipitated phases migrated to the center of Fe-rich spherulites due to the interface energy gradient and density difference. The Fe-rich spherulites presented different morphologies because of the differences in composition and cooling rate. The macro-segregation mechanism of melted unmixed zone near fusion boundary was studied based on solidification theory of fusion-boundary. When copper filler wire was used, non-isothermal pool boundary formed at fusion boundaries. The liquidus temperature of bulk weld pool is lower than that of steel base metal in joint. Resultantly, a bulk molten pool cooler than the melted 316L base metal formed near the solidification front. Subsequently, the melted unmixed BM was carried into the cooler molten pool by the convection and agitation, and solidified quickly into a peninsula and island shape before it fully mixed with the bulk weld pool. A softened zone existed in the copper sheet, which consisted of zones with different grain sizes. All tensile specimens fractured at HAZ on the copper side with a ductile fracture mode, and the maximum strength reached 81.7% that of Cu base metal.