The mechanical properties and microstructure features of the fine-grained heat-affected zone (FGHAZ) of ASTM4130 steel was investigated by optical microscope (OM),scanning electron microscope (SEM),transmission electron microscope (TEM),and welding thermal simulation test.It is found that serious embrittlement occurs in the FGHAZ with an 81.37％ decrease of toughness,compared with that of the base metal.Microstructure analysis reveals that the FGHAZ is mainly composed of acicular,equiaxed ferrite,granular ferrite,martensite,and martensite-austenite (M-A) constituent.The FGHAZ embrittlement is mainly induced by granular ferrite because of carbides located at its boundaries and sub-boundaries.Meanwhile,the existence of martensite and M-A constituent,which distribute in a discontinuous network,is also detrimental to the mechanical properties.