Abstract: Welded joints of 10MnNi2MoV low alloy steel pipe in service in a power plant was taken as the research object, whose surface crack location, morphology, microstructure and fracture were studied and analyzed systematically. The results showed that serrated cracks were easy to produce in ЭА-395/9 surfacing layer near the cosmetic weld of welded joints of 10MnNi2MoV steel pipe, which usually expanded along the growth direction of columnar crystals. At the same time, it was easy to produce strip and granular precipitates, T-shaped grain boundaries and other microstructure and features in migration grain boundary near the surface of the surfacing layer. These microstructures provided internal conditions for formation of ductility dip cracking of welded joints of 10MnNi2MoV low alloy steel pipe. For welded joints of low alloy steel that have been in service for a long time in thermal power plants, ductility dip cracking is easy to occur in the surfacing layer of welded joints, and once the crack is formed, it will expand rapidly, which seriously threatens safe operation of power plant units.