Abstract: To estimate the residual life of pipeline welds, this paper discuses the mechanism of non-penetrating crack growth and the technique to predict the life of weld seam with defects. Accordingly, a dynamic method to calculate the residual life of weld seam is proposed, on the basis of the build-up method and the failure assessment diagram (FAD) indicated in BS 7910-2005, as well as the non-penetrating crack growth model. The C# programming is successfully applied to achieve a specific algorithm and also calculate and analyze how the non-geometric parameters of weld defects of an oil and gas pipeline affect the life of weld seam. The calculation results indicate that the type of defect (long or short), i.e. the ratio of the depth and length of defect, has significant impacts on the number of cyclic loading to be accepted by the defect. In case of short defect, its number of cyclic loading will reduce gradually as a function of relative depth. In case of long defect, its number of cyclic loading will decrease and then tend to be stable.