This　study　was　focused　on　welding　joints　of　reduced-activation　ferritic/martensitic　(RAFM)　CLF-1　steel　medium-to-thick　plates　produced　via　laser　beam　welding　(LBW)　and　electron　beam　welding　(EBW).　Such　joints　were　used　in　the　ITER　project,　in　particular,　in　the　back　plate　mockup　of　Helium　Cooled　Ceramic　Breeder　Test　Blanket　System　developed　in　China　in　2015.　The　microstructural　evolutions　and　the　mechanical　properties　of　fusion　zones　(FZs),　weld　zones　(WZs),　and　heat-affected　zones　(HAZs)　of　LBW　and　EBW　joints　were　analyzed　and　compared　using　electron　backscatter　diffraction　(EBSD)　and　transmission　electron　microscopy　(TEM).　Moreover,　the　microstructural　differences　between　the　two　types　of　joints　and　their　effect　on　the　impact　properties　were　discussed.　Results　showed　that　the　well-formed　welds　without　defects,　such　as　pores,　incomplete　fusion,　and　cracks,　could　be　obtained　via　LBW　and　EBW.　Different　heat　inputs　led　to　the　differences　in　the　grain　size　gradients,　the　boundary　angles,　the　content　of　carbides,　and　the　distribution　of　dislocations.　Despite　obvious　differences　in　morphology　among　FZ,　WZ,　and　HAZ,　both　types　of　joints　exhibited　excellent　tensile　properties　and　impact　toughness.　The　tensile　strengths　of　the　LBW　and　EBW　joints　were　633　and　634　MPa　at　room　temperature,　respectively,　versus　370　and　366　MPa　at　550　degrees　C.　The　mean　impact-absorbing　energy　values　of　LBW　and　EBW　joints　were　251　and　284　J.　The　further　analysis　revealed　that　the　low　delta-ferrite　content,　small　effective　grain　size,　large　grain　size　gradient,　high　grain　boundary　angle,　diminutive　size　of　M23C6　carbides,　high-Ta　content　in　MX　carbides　and　low　dislocation　density　led　to　high　impact　toughness　of　the　LBW　joint,　while　the　microstructure　of　EBW　joint　failed　to　show　the　improvement　of　impact　toughness,　and　the　extraordinary　impact　toughness　could　be　attributed　to　the　high　scattering　band.　(C)　2021　Elsevier　B.V.　All　rights　reserved.