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• An UNSM system is used to fabricate hierarchical surface structures on metals. • The formation of nano-scale structure originates from pile-up produced by dynamic indentation. • Dynamic indentation produces higher pile-up as compared with quasi-static indentation. • Dynamic indentation induces high resistance to dislocation motion, forcing material to pile up. Ultrasonic-assisted surface texturing can produce surface textures with hierarchical micro/nano-scale structures on metal components. In this work, we prove that the formation of nano-scale structure originates from pile-up produced by ultrasonic dynamic indentation. Both quasi-static and dynamic indentations are performed on pure copper, 316 stainless steel, and pure titanium using an ultrasonic nanocrystal surface modification system. For the first time, it is revealed that ultrasonic dynamic indentation produces significant higher pile-up as compared with that of quasi-static indentation, making nanoscale structure possible. Finite element simulations demonstrate that dynamic indentation produces larger equivalent plastic strain, an indicator for work hardening, in the region close to the edge of the contact interface. It is conjectured that higher degrees of work hardening and dislocation density result in a higher dislocation resistance as compared with quasi-static indentation. The plastic deformation is confined at the contact interface and is then forced to move upward, leading to a significant pile-up. [ABSTRACT FROM AUTHOR]