Abstract: The rule for assessing the lightening of welded frame of Bo-Bo locomotive bogie is proposed. Based on the analysis result of the frame s whole structure with finite element method, the stress distribution in the lateral butt-weld on the bottom cover plate of the side beam of the frame is analyzed with boundary element method. The stress concentration due to the incomplete penetration root of the weld is discussed, and a feasible scheme to reduce the stress in the tensile butt-weld is presented. A comparison between the results of the theoretical analysis and fatigue intensity tests shows that the stress distribution in the weld region can be calculated accurately with boundary element method.