Abstract: This paper investigates the effect of welding defect on fatigue life of butt welds of an ultra high strength steel (30CrMnSiNi2A) and gives a quantitative approach to evaluate fatigue resistance of the butt welds containing welding delect. The results indicate that the welding defect can be devided into two kinds, the crack-like defect and noncrack-like defect. The former plays a role as a crack and the total life of a welded joint with it is approximate to the crack propagation life, i.e. the time of crack propagation from the initial detect size to a critical one to induce final fracture. The latter, however, can be tnken as a notch and the life of the butt weld containing it is the sum of the fatigue crack iniiiation(FCI) life and the fatigue crack propagation (FCP) life. The FCI life can be predicted by using the FCI life expression determined from notch specimens of material, but modification should be made to take account of the difference surface conditions of the notched specimen and the real FCI location in the welded joint. The FCP life can be estimated by using fracture mechanics approach. Finally, several methods to approve the fatigue resistance of butt welds are proposed.