Abstract: According to AWS D1.1M-2010 and ISO 15653:2010, the crack tip opening displacement (CTOD) test and the Charpy impact test of TOP-TIG welded joints made by 9Ni steel using in liquefied nature gas (LNG) had been conducted. The microstructure, X-ray diffractometry and fracture surface were performed to study the low-temperature toughness of welded joints for TOP-TIP welding on 9Ni steel. The results revealed that the impact toughness of the welded joints was very good and much higher than the standard requirements; The CTOD characteristic values were higher. The microstructure of HAZ was mainly consisted of lath martensite with more dislocation. The fracture surface of HAZ showed quasi-cleavage fracture with shallow and small dimple. The microstructure of the welded seam comprised of austenite dendritic structure and other equiaxed crystal had excellent fracture toughness which with a significant number of dimple fracture surface. The low temperature toughness of welded joints for TOP-TIG welding on 9 Ni steel was good enough to reach the engineering requirements.