Abstract: It is necessary to establish an acceptable level for possible defects in steel structures on the basis of "fitness for purpose".In accordance with BS 6235,code of practice for fixed offshore structures,for a given stress or strain level the COD test can be used to determine the maximum tolerable flaw size using the procedure proposed in PD 6493.Besides the equations of fracture toughness,defect and applied strain,this code also recommends a tolerable defect parameter α_max=B/4 for a thickness of B≤50mm.In this paper the allowable flaw size in welded joint specified by BS 6235-82 is examined by means of wide plate test for two steels A131 and A537,which are commonly used for offshore structures in China.Tes-ting technology is described in detail and the useful conclusions are given also,i.e.,the strains corresponding to fracture are 5.57% for A131 steel and 4.17% for A537 steel respectively at -40℃.They are higher than the strain predicted by the equation for a wide plate under conditins: SCF (Kt)=10,design stress σ=0.6σ_s,and preloading stress σ_P=0.8σ_s.