Abstract: TA15 titanium alloy and 18-8 stainless steel were welded by tungsten inert-gas arc welding with copper alloy as filler metal. Microstructure, phase constituents, elements distribution and microhardness near the fusion zone of TA15 alloy/18-8 stainless steel joint were investigated by optical microscope, scanning electron microscope, electron probe microanalysis and microhardness tester. Results indicated that the molten unmixed zone near TA15 alloy side primarily included α-Ti, brittle phosphide (Ti₃P) and Ti(Cu, Fe), while the fusion zone near 18-8 stainless steel was mainly composed of Fe-P eutectic microstructure. The enrichment of P in the fusion zone near 18-8 stainless steel contributed to the forming of eutectic microstructure. The residual Cu formed the clump zone. The microhardness of fusion zones was obviously higher than that of heat affected zone and weld metal. Brittle Ti₃P phase resulted in the highest microhardness of molten unmixed zone near titanium alloy reaching 720 HV_0.5.