Abstract: Objective This study aims to quantify impact of incomplete penetration defects caused by welding processes on fatigue strength of 16MnDR butt welded joints. Fatigue performance differences between welded joints with incomplete penetration defects and no defects were investigated. Methods Fatigue tests were conducted on both types of butt welded joints with an electromagnetic resonance high-frequency fatigue testing machine under a stress ratio (R=0.5). The probabilistic fatigue strength at 10⁷ cycles for welded joints with incomplete penetration and no defects was compared. Additionally, failure analysis of fracture under SEM was performed to identify fatigue crack initiation points and causes of fatigue strength reduction. Results Fatigue test results revealed that fatigue strength of butt welded joints with incomplete penetration defects was only 17% that of butt welded joints with no defects. Failure analysis showed that fatigue cracks in butt welded joints with incomplete penetration originated from incomplete penetration fusion zone of root, where geometric stress concentration and initial defects formed during welding were the main reasons for fatigue strength reduction. Conclusion Based on finite element analysis results and fatigue test data, crack propagation threshold for butt welded joints with incomplete penetration under a stress ratio (R=0.5), considering residual welding stress effects, was estimated. A relationship between fatigue limit strength and size of initial incomplete penetration defects was established. The study provides important insights for optimizing welding processes and improving fatigue strength of welded joints.