Abstract: When CTOD (crack tip opening displacement) tests for welded joint of offshore-oil-platform steel EH36 were performed at -15℃ according to test standard BS7448, phenomenon called pop-in appeared in weld and heat-affected zone of many specimens which had been prepared by single or twin wire submerged arc welding respectively. To this problem, in order to accept or reject all of pop-in effects occurred in welded joint reasonably, variable value on F-V (F:load, V: displacement) curve was measured and calculated, and mierostrueture around the initiation point in fracture plane was examined under optical microscope according to correlative assessing standard in BS7448 Part Ⅰ. The study is very important for calculating CTOD value properly, assessing fracture toughness of welded joint accurately and constituting a welding condition rationally and effectively.