Abstract: Bending creases appeared on the DC51D+AZ hot-dip Al-Zn alloy coating thin plate in the process of bending forming. Chemical composition, microstructure and mechanical properties of normal product were contracted with those of defective product in order to find the causes of the defects. The results show that lower ultimate rolling temperature and coiling temperature were the main reasons for the defects because they refined the grain size of the thin plate, increased the hardness and the yield elongation and reduced the stability of plastic deformation. Consequently, it is proposed that the manufacturer raise ultimate rolling temperature and coiling temperature and enough annealing to produce the DC51D+AZ hot-dip Al-Zn alloy coating thin plate.