Abstract: Mg alloy casting parts commonly suffer from drawbacks of low surface properties, high susceptibility to corrosion, unsatisfactory absolute strength, and poor ductility, which seriously limit their wide application. Here, a surface nanocrystallization technique, i.e., ultrasonic surface rolling (USR), was applied on an as-cast AZ91 Mg alloy sheet to improve its corrosion resistance and mechanical properties. The USR produces double smooth surfaces with Ra 0.036 μm and gradient nanostructured surface layers on the sheet. Due to this special microstructure modification, the USR sheet exhibits 55% and 50% improvements in yield strength and ultimate tensile strength without visibly sacrificed ductility comparable to its untreated counterpart, as well as a 24% improvement in surface hardness. The USR sheet also shows good corrosion resistance in 3.5wt% NaCl aqueous solution. The corrosion current density of the USR sheet reduces by 63% after immersion for 1 h, and 25% after immersion for 24 h compared to that of the untreated counterpart. The enhanced strength and hardness are mainly related to the gradient nanostructure. The improved corrosion resistance is mainly ascribed to the decreased surface roughness, nanostructured surface, and residual compressive stress. The present results state that USR is an effective and attractive method to improve the multiple properties of Mg alloy casting parts, and thus can be used as an additional and last working procedure to achieve high-performance Mg alloy casting parts.