Abstract: British standard 7910 was applied to engineering critical assessment for welded joints of the API5L X56 pipeline steel with surface flaw at the weld toe.The failure assessment diagrams were derived from the tensile test results.In accordance with the BS 7448 Part 4,fracture toughness test standard,δ-Resistance curve tests were conducted with multiple specimens at(5℃) in welded joints of X56 pipeline steel.And then the δ_0.2 values of weld and HAZ were obtained.The assessment was carried out by using the maximum stress at different water depth,and the stress concentration and residual stress were also considered.The tolerable surface flaw sizes were gained at perfect alignment and maximum allowable misalignment.This study lays the foundation for judging the acceptability of flaws.