Abstract: It was effective to resist precipitation of grain boundary ferrite and make fine homogeneous acicular ferrite formation by transferring suitable amount of Ti-B elements and suitable amount of rare earth elements so that it increased low temperature toughness of weld deposited metal.It had been found that weld alloy elements could be the nucleating-centers of acicular ferrite as long as they formed fine non-metallic inclusions which were difficult to dissolve by the investigation of transmitted electron microscope.Titanium could form such fine combination nucleus(TiO) which spreaded in the weld metal and was acicular ferrite nucleus forming.Acicular ferrite consisted of a lot of substructure,and it was the microstructural reason to improve the low temperature toughness of weld deposited metal.