Abstract: Microstructures of the pressure boundary welded joint of CAP1400 steam generator were studied with optical microscope and scanning electron microscope. The pressure boundary joint is made up of SA508Gr.3Cl.2 steel base metal, heated affected zone, surfacing layer and butt weld. The results show that the base metal of the joint had a fine tempered bainite microstructure and the heat affected zone had a coarse martensite microstructure. The microstructure of the stainless steel surfacing layer was mainly composed of columnar austenite and little ferrite, and that of the nickel based alloy surfacing and butt weld mainly consisted of columnar austenite. Austenite recrystallization occurred both in the stainless steel surfacing layer and in the nickel based alloy butt weld because of the thermal cycling effect of the multi-layer and multi-pass welding. The carbides precipitated in the austenite matrix.