Abstract: The influence of MIG welding heat input on the microstructure, expecially ferite content, and hardness of 2205 duplex stainless steel plate weldments was investigated for single and multipass bead-on -plate and cruciform fillet weld joints and multipass butt welds.Charpy impact values from 0 to -100℃ Were also measured on the butt welds. For the fillet welds, pure argon was compared with argon-30% helium-1% oxygen as a shielding gas,from the viewpoint of weld metal penetration and bead shape.Ferrite contents were determinedby computerised image analysis and were compared, for the parent plate omly,withindependent measurements made using the method of point count metallography.The results shown that 2205 duplex plate can be MIG welded to give weldments which have excellent toughness, with satisfactory hardness and microstructures. The highest heat input made the Charrpy values decrease remarkably only at -100℃.The mixed shielding gas gave deeper penetration and better bead profile than the pure argon. Computerised image analysis gave fast, accurate and reproducible ferrite content determination.