Abstract: The micromorphology and micro-area chemical composition of five invisible zinc dross defects in hot dip galvanized sheet were studied by scanning electron microscope and energy spectrometer, and corresponding control measures were proposed. The results show that the invisible zinc dross defects in the hot dip galvanized sheet includedrich Al-Zn type, rich Fe-Al type, rich Fe-Zn type, residual Fe type and ZnO type. Rich Al-Zn zinc dross defect was loose shaped with color of grey and white; rich Fe-Al zinc dross defect was needle shaped or platy shaped with the color of off-white and distributed dispersively; rich Fe-Zn zinc dross defect was mixed with zinc coating or embedded in the zinc coating in the form of granules; residual Fe zinc dross defect was linear or flaky with color of black or dark grey, and the distribution was centralized; ZnO zinc dross defect was formed by ZnO embedded in zinc coating with continuous platy shape and distributed dispersively. After taking measures of high-aluminium control technology, equipment optimization and adjustment, dew point controlling and termly cleaning zinc ash, the overall average repair rate of the hot dip galvanized sheet was reduced from 40% to 6.88%, and the surface quality was improved obviously.