Abstract: Based on BS7448 fracture toughness test experiment standard (ISO/TC164/SC4-N400)and DNV-OS-C401, crack tip opening displacement (CTOD) tests are carried out on 80 mm thick plates butt joints of offshore petroleum platform joined by the submerged-arc welding and CO₂ gas shielded welding.The CTOD values of weld metal and the heat-affected zone are tested at 0℃. The possibility of avoiding post-weld heat treatment in thick welded joints is discussed using the submerged-arc welding and CO₂ gas shielded welding.These work provide the scientific basis on the offshore platform construction.