Abstract: Defects would be play important influence on the quality of hot galvanized steel sheet used for cars. A method was shared to analyze the defect reason by using electronic scanning technology and micro characterization of the chemical composition in defects. Application methods of physical testing and chemical analysis technology in the production of hot galvanized steel sheet used for car were summarized. Results showed that the defects were generated between galvannealing furnace and skin level section, and the chemical composition analysis determined that the black spots were rubber material. Through investigation on the line, it was found that only a wringer roll on the quench tank was made of rubber material between the galvannealing furnace and skin level section. Finally the defects were removed by changing the roll, which ensured the normal production.