Abstract: The joining of Mo-Cu alloy with 18-8 stainless steel was carried out by tungsten inert-gas arc welding with filler wire. Microstructure, microhardness, element distribution and phase constituents near the fusion zone of Mo-Cu alloy/18-8 stainless steel joint were investigated by using optical microscopy, scanning electron microscop, microhardners and X-ray diffraction method. Results showed that there were mixed phases of martensite and austenite in the fusion zone at side of Mo-Cu alloy and austenite-ferrite dual phase in the weld metal and fusion zone near 18-8 stainless steel. The formation of carbonized layer in the fusion zone was promoted by the diffusion of Mo from Mo-Cu alloy to weld metal. The phase constituents near the fusion zone at side of Mo-Cu alloy were Mo, Cu, γ-Fe(Ni), Fe_0.54Mo_0.73 and Cu_3.8Ni. The existence of carbonized layer and Fe_0.54Mo_0.73 intermetallic compound resulted in a higher microhardness of 1200 HV.