The microstructure of die-chill skin on a die-cast AZ91D plate has been characterized, and its effect on the formation and corrosion resistance of stannate conversion coating has been investigated. The die-chill skin contained interconnected pores, around which carbon-silicon-containing magnesium/aluminum oxides were detected. The stannate conversion coating on the die-cast AZ91D consisted of hemispherical magnesium tin oxide particles. When compared with the substrate whose die-chill skin had been removed by mechanical polishing, the presence of die-chill skin enhanced the nucleation of magnesium tin oxide particles and reduced the thickness of the conversion coating layer. The intact sites on the substrate with die-chill skin, however, led to the formation of uncoated spots after 10 min of stannate conversion coating treatment. The thinner conversion coating containing uncoated spots was less effective in retarding the reduction of water during cathodic polarization in a mixture of 0.1 M sodium sulfate and 0.05 M sodium chloride solution. These uncoated spots, which were resistant in the alkaline stannate solution, seemed to prohibit the oxidation and dissolution of the substrate during anodic polarization.