This paper reports for the first time the exact mechanism of the selective metal nanoscaleetch method (SMNEM), which is a simply controllable and cost-effective approach formetal nanoscale etching and size reduction. This method is based on the galvanicdisplacement, Kirkendall effect, and selective etching. The size reduction is linearlycontrolled by the galvanic displacement in the early stage (1 min). In the later stage, aKirkendall void is formed between the Ni and Au layer, which is well understood fromFick’s law of diffusion. As matching with the experimental results, Ni nanowires with30 ± 4 nm widthand 50 ± 7 nm height were easily fabricated from nanostructures with150 ± 3 nm widthand 110 ± 2 nm height. The morphology and position of the Ni nanostructures are all predetermined bytheir initial conditions. Also achieved was the fabrication of complicated three-dimensionalnanostructures such as Au nanowires and nanochannels from reduced Ni nanowires by afull replacement reaction.