Abstract: One-side welded butt welds are widely used in welded structures. This paper gives the results of tests conducted on low carbon steel butt welds containing incomplete penetration defects under fluctuating tensile stress. As the incomplete penetration defects generally occur at the surface of weld in the case of one-side welded butt weld, the loss of fatigue strenght is more severe compared with that of double-side welded butt weld of the same defect ratio. Inclusions existing in the tip of incomplete penetration may intensify the stress concentration in the stress concentration zone. This further lowers the fatigue strength.