With the increasing attention on the quality of electric power, government has setup strict regulations on the equipments of power system, including transformer, gas-insulated metal-enclosed switchgear(GIS), fuse cutout(CO), lightning arrester(LA), etc., to ensure the transmission apparatus possessing high reliability to provide better quality of electric power. LA is the front end protection for surge overvoltage in all transmission and distribution lines. It is also the best defender to protect the back-end equipments. Therefore, this paper uses 9kV defective zinc oxide arresters(ZnO) for testing. By partial discharge(PD) method, according to standard, IEC60099-4, the PD patterns of defective products has been explored. The results of this study could be used as reference for manufacturers to find the causes of the defects of their products.