Abstract: X100 pipeline steels were welded by submerged arc welding and the the microstructure of base metal, weld seam and heat affected zone (HAZ) of X100 pipeline steel were observed by optical microscopy. The results show that the microstructure of X100 pipeline steel composed of ferrite and granular bainite, and the fine granular bainite diffusely distributed at the ferrite grain boundaries and in ferrite inside. The main microstructure of weld seam was acicular ferrite, granular bainite, a small amounts of M/A island and black inclusions particles. The forms of ferrite were different in different position of heat affected zone, the form of ferrite in coarse grain area was quasi-polygonal ferrite and polygonal ferrite, and which in fine grain area was bainitic ferrite.