Abstract: The measuring and testing technique of large sized crack tip opening displacement (CTOD) tests for welded joints used in the construction of offshore petroleum platform under low environmental temperature (-18℃) were performed in accordance with the BS 7448 standard (Part.2,1997).The CTOD tests on the weld metal and heat affected zone(HAZ)regions were conducted for the welded joints with a thickness of 2.5in (63.5 mm) welded by SMAW and SAW technology respectively.The testing results and some technique problems during the test procedure were discussed in details.Except for two specimens of HAZ,it is shown that the CTOD values of all specimens meet the requirement of minimum CTOD 0.15 mm proposed by the Phillips.The experimental results provided the basis to estimate the effect of heat treatment procedures after welding on the welded joints with large sized thickness.