Abstract: With the application of failure assessment diagram(FAD) approach provided by 'structural integrity assessment procedure for european industry'(SINTAP),Class II failure assessment curves were set up for two under-matched joints of 10CrNi3MoV steel with different weld toughness.Based on the tensile test and CTOD test,safety assessment for the two joints was carried out.It showed that after the assessment level being selected,FAD shape changed very little.The results indicated that weld toughness was weakly influenced on the joint's resistance to plastic instability,but it can significantly influence on the anti-brittle capacity.The higher weld toughness is,the more likely assessment point locates within failure assessment curve,i.e.the joint's safety performance is better.