Abstract: A guard board for vehicle cooling device fractured at the position for fixing nuts. Reason for fracture was analyzed by the methods such as microstructure analysis, fracture morphology observation and chemical composition detection. The results show that the guard board factured in a fatigue manner. Two fatigue crack sources existed on fracture. One was located in weld fusion zone of internal surface of the guard board and the nut, and the other was located in welding heat affected zone on outer surface. The crack propagation by alternating loading led to fracture of the guard board. Welding defects were the main cause of fatigue cracking.