Abstract: The influence of heavy rare earth element on the weld metal of low alloy steelhas been investigated by means of scanning electron microscope (SEM),transmittedelectron microscope (TEM) and X-ray energy dispersion spectrometer (EDAX).Theresults showed that heavy rare earth element can be transferred into the weld metalup to an amount of about 10 ppm by adopting appropriate prescription of flux coat-ing and welding procedure.The rare earth element reduces the area density and areapercentage of inclusions and increases appreciably the low temperature toughness of weldmetal.There is a good relationship between the decreasing of amount of inclusion andthe increasing of toughness.So the main reason for the low temperature tonghnessbeing improved can be attributed to the cleaning and modification of the weld metalby adding rare earth.The rare earth element mainly concentrates in the silicate inclu-sions containing Ti and Mn,in which the rare earth content is hundreds times morethan its average in the weld metal.