Abstract: The GTAW (gas tungsten arc welding) was employed to weld Inconel 690 nickel-based alloy overlay plate and S32101 duplex stainless-steel, a new type material for fabricating spent fuel pool, with lap joint. Microstructures and mechanical properties of the joints fabricated with autogenous welding and filler welding were comparative studied. Compared with autogenous welding, a sound bead, with larger lap width and the tensile strength of 538 MPa, was obtained by filler welding under the optimized welding parameters. The welds were characteristic of cellular grains and fine ferritic precipitates dispersed among them. Different solidification microstructures were presented at fusion boundaries within weld. Epitaxial growth can be observed in the side of Inconel 690 and planar growth in the other side. Austenite dissolution range in the heat affected zone of the filler welding is narrower than that of autogenous welding.