Abstract: There exist three brittle temperature ranges during the course of solidification and cooling process in the weld metal of Low-carbon steel. The first range is the solidification brittle temperature range (BTR_S). Sulfur is the principal element to widen the BTRs. A relationship among the sulfur and manganese in the world metal and the BTRs has been established as follows.BTR_S(℃)=25+927(%S)-28(%Mn)The second range is the ductility-dip brittle temperature range (BTR_D), which is about 1100~800℃ for the tested materials. A mechanism of the formation of BTR_D is proposed. The third BTR is below 600℃. Its formation mechanism needs further investigation.