Characteristics of high power LASER welded carbon steel pipe has been investigated. The heat-treated seam weld portion has the excellent low temperature toughness and the sour resistance. This excellent quality is due to the combined effects of LASER welding and its manufacturing process which means low welding heat input , same chemical composition of weld nugget as mother plate and in-line heat treatment on weldment. Keywords: high power LASER welding, linepipe, low temperature toughness, HIC, SSC